Question: Does he puke in the car if watching a video? If not, it may have been a fluke. I can read on a plane, but not in a car. However, if the window shade is up and I'm trying to read during take off, well..... no beuno
As for feeling like you shouldn't go because of the potential stresses (both for parents and kiddo).... Remember all the stress you felt preparing for your last trip when he was just a little one? Car seats and airplane rides and hotels and sleeping arrangements and pack-n-plays vs hotel cribs... And then you were there, and things went well enough that you decided to go back 2 years later.... And here we are. Yes, the stresses are different, but the potential for amazing memories is also right there! And at 3, your son will be old enough to have lasting memories from this trip, which is kind of awesome in my opinion!
Our family doesn't have an autism diagnosis. In our case, our daughter (who we adopted as a toddler) was born drug addicted to several substances. We worried early on about autism--she didn't seem to have a tight bond with anyone (seriously would have let the man on the corner take her home), she was very particular about routines, reacted with hostility to any variation from how she expected things to go, some spinning with arm flapping, some minor tactile issues (seeking different textures to touch to calm herself, only able to tolerate wearing elastic-waisted leggings), along with abuse issues that occurred from before she was with us.... At any rate, my point is that we weren't/aren't dealing with a neuro-typical child either, and, for all the stress I had planning and taking her each time, she still has only great memories of it.
Disneyland is pretty darn accepting of those who aren't "the same" as "everyone else." In our case, our daughter was easily overstimulated. So, by day 3 on our first trip, she had been at Disneyland for like 24 hours (over those 3 days), was tired of walking, had neglected to tell me about the 3 painful warts on her foot (she finally told me there was an owie on like day 5), and Innoventions "wasn't a ride" so she refused to have anything to do with it. Until she found Fruit Ninja on the game console in the House of Tomorrow or whatever it used to be called downstairs.... Then, she didn't want to leave. And hell hath no fury like a child who is big enough to resist leaving when she's not ready. Eventually, I realized she was over-stimulated, and that, whether she believed it or not, we needed to go find a quiet snuggle place to regroup. Preferably one where she also had room to spin, and no one would look at her weird for having a blanky and sucking her thumb until she could get calmed. So, I was carrying a kicking, punching, screaming, biting child out of Innoventions. Of course I got some looks; at this point, we always got looks. I also got a few parents that locked eyes with me and gave me a knowing, understanding nod; it was nice to know that my daughter wasn't the only child, and I wasn't the only mother, who experienced full on meltdown mode in the middle of the happiest place on earth. Cast members attempted to help, but I let them know we just needed to get some space, and the only help I needed was someone opening the door so we could get her to a quiet spot.
If he's a screamer in some enclosed places, then I'd go ahead and ask for a
DAS, as well as a "stroller as a wheelchair" tag (assuming he's cool with his stroller). That will allow access to wait in a different area (not enclosed, not in the sun [or, in the sun if that makes him happier!], not confined in an unfamiliar area) until it's time to get on the ride. And if he screams throughout the ride, I promise, it's not a big deal. Yes, you will sweat it out, you will be uncomfortable, perhaps even a bit embarrassed (I was!), but if a guest complains about it to the cast member, they will just give that party another ride through. So, you aren't really "impacting" them as much as you may worry that you are.
I know someone else mentioned perhaps doing a "mock flight" and several different airlines do this at different places and different airports. I would call the airlines directly and ask about it, as it is organized through them. The airline I remember hearing about doing it was Southwest, I believe. Of course, you also need to balance that with any potential anger/upset that they did "all that" and didn't get to fly (which would have totally ticked my daughter off, and we would have had a full meltdown on the plane when it didn't take off! If they don't do this at your local airport, perhaps you can do a social story ("what to expect" story) about it. I know they can do gate passes, so maybe see if you can take some pictures and write a little story to go with it. These stories helped my daughter a lot, because she knew what to expect next. You can make them as short (or as long) as he is willing to look at the pages. Usually, we tried for 10 pages with only 1-2 thoughts per page ("Grandpa will drive us to the airport in his car. We will go on the freeway." or "We will go by safety police officers with police dogs. They may sniff us, because the doggies wonder if we have doggies or kitties at our house.").... Or even just maybe doing pictures/icons for what to expect in what order? I'm just thinking that our daughter does so much better, even when she couldn't/can't express it, when she knows what to expect!
Good luck!