First, you sound like a wonderful person - very caring and it really says a lot about you that you're trying to figure this all out.
I'm just going to give you a few random stories, and maybe something will help in some way.
My parents were divorced when I was 12, and they both made it very easy for my brothers and I when it came to holidays. 'whatever works' was their motto - both didn't care if they saw us on "the day" of the holiday. We actually got 2 very nice christmas's, 2 very nice thanksgivings, etc. It never even felt like it wasn't "the day". In fact, having to still split holidays (my family and dh's family), my family has been doing thanksgiving at my house the weekend before thanksgiving for a few years now, and it works really great! The Saturday before thanksgiving is my family's thanksgiving. We're then all free on the real thanksgiving to do in-laws, and my mom and stepdad go out to dinner (my dad's out of the picture, but if he weren't, we'd simply have 3 thanksgivings and enjoy them all equally). Splitting the holiday never works... the people who have to go 2 places (pp's neices) feel pressure and stress and cannot enjoy the holiday, and the people who are hosting always feel slighted. So your dh never getting to see his ds on "christmas morning" shouldn't matter (i know it stinks, but it's not important) - I'm sure you did christmas on another day w/ ds. "the day" isn't important to the child.
I bring this up because my dh's parents are also divorced, and his mom was no way/no how giving up the 5 kids for christmas (since dh's dad was the one who had a girlfriend and left). So on Dec 25, dh's dad would drive from an hour away, park in the driveway, the kids would come outside to his car and pile in, and sit in the driveway for 20 minutes opening gifts he brought. That was their christmas w/ their dad. Sad, isn't it?!?? I think the dad was SO wrong doing that - but he was determined to see his kids on "the day" of christmas. The kids have not one memory of a nice 'christmas' with their dad after the divorce, and they all think the car thing was ridiculous and sad for all of them. DH and I were recently talking about this, and I asked if his dad ever invited the kids down to his house over christmas break for a christmas with him. Have stepmom (or dad, not being sexist), make a big christmas dinner and do a whole day of christmas, just on Dec 28th or so. There's always a whole week to work with. DH said no, and said that would have been nice instead of a 20 minute car thing.
I also didn't post this before, but thought as I've been following this thread, that your party for him the weekend after might have been too soon and he might still be partying and hanging out w/ friends. That can last a few weekends, not just the weekend of graduation. And teens don't want to give up any time from friends to do any family things - whether it's the family they live with or the family they don't. And I do feel bad for any divorced kid, having to make time on their weekends to see the other parent. It's so not fair to the kid to have to cut time into what should be their social time, to see another whole family. It's a fact of divorce and life, but it still stinks for the kid.
My dh also has 2 half sibs from his dad leaving his mom, getting remarried and having 2 more. The first 5 have never felt anything for the second set of kids - they "like" them well enough... but love... not really. It's because they didn't live with them. Not living with someone as you're growing up really makes a difference in the bond you form. Of course, there were also hard feelings because the stepmom was the woman the dad left the mom for - then had kids with - so a totally different situation from yours. But your dd's grew up 'having' a brother, even though he didn't live there. But ds didn't grow up having sisters... it's different.
My sil has an 18yo ds from her first marriage, and now has a 1yo and 2yo w/ my brother. She couldn't figure out why her first ds didn't embrace his new siblings, and she was mad when he didn't come over to see them for months after they were born (her ds lived w/ his dad). In talking about it at a family dinner, my dh chimed in how the sibling situation was similar to his w/ his 2 half sibs, and everyone in my family has always understood how/why dh isn't close to his half sibs. That opened my sil's eyes a lot (all of us had never really thought of it that way, because we were on the other side of the fence this time). She then dropped the whole "these are your brothers" thing w/ her oldest ds, and offered to just meet him alone, just him and her for dinner, etc. That worked better. In sil's mind, having her 3 boys all be close would be ideal, but she now realizes it's just not going to happen, at least not now.
DH's father has always tried to be like that too with all the siblings (everyone be close and love each other), and it pisses off the first siblings to no end. They don't like hearing "your brother or your sister" when referring to the 2nd set of siblings. They just don't *feel* it. I'm not saying it's this way with your family, just giving food for thought.
Also, with your dh and ds seeing each other (or all of you guys), maybe meeting him at a time that isn't cutting into his weekends would be better for now, with the stage in life he's in. Meeting on a weekday at a pizza place for dinner might be less pressure for ds to fit into his schedule than a weekend day. And i'd recommend for now dh and ds having a few visits alone, then maybe all of you.
Sometimes 'regular' families aren't easy, especially with teens... a blended family has even more challenges to work around. Especially with the whole 'making time for the other family' thing... the family who lives with the child gets to see him for breakfast, after school, dinners, before bed, etc, so no "special" time is needed. Lives are busy enough today for everyone, then add in making special time for a 2nd family... it's a lot on a kid. My brothers and I lived w/ my mom, and when we were teens, I didn't "do" anything with my mom, because I lived with her. But having to make time to go "do" stuff with our dad was hard sometimes, even if "doing" was just going to his place to see him. It was something else in our schedule. But dad made it easy... he always lived w/in a few miles of us, and over the years gave us various jobs to make a little money and be able to see him (he had a business so that was easy for him to do, but I also cleaned his apartment w/ one of my friends too). We also had card night 1 night a week when we were older teens (pinnochle - sp????) with dad and stepmom, which we really enjoyed. Something ds really enjoys doing might be a nice way to spend time w/ him (going to local ballgames, or bowling, or fishing, etc), as a regular monthly thing maybe. Then fitting in dinners when he's available, and going 'to him' instead of him coming to you maybe for now.
Being a stepchild is SO hard for some kids, and for the family you're not living with, it's almost impossible to feel like you're *really* a part of that family, no matter how hard the parents (you and your dh) try to make it that way. If you don't wake up to these people every day, and wake up in this house, it's just going to be different. So while you and dh and your 2 dd's open your arms to ds when he comes over, and always have a place for him in your house and family and hearts, I bet he doesn't feel the same comfort level fitting into that place you've made for him, as you guys feel making it. It's like being invited to a friends house for dinner... the friends family is totally comfortable having this familiar face at dinner with them, this 1 extra person... but the invited guest, even if very friendly with this family, isn't "as" comfortable as the other people are. Certainly not as comfortable as being in 'your own' home is, knim???
Maybe none of this is helpful - but at least wanted to say you sound like a very caring person, and i'm sure it's hard being a stepparent, but it's 'really' hard being a stepchild for some kids, and it sounds like your ds might be one of them. Hope this all works out, and keep us updated.
