Kids are really spoiled nowadays. I don't mean yours in particular, I mean all of them (including my own)
I would be more concerned actually that he now has the expectation that he is going to get a big trip for every birthday. He's only 6 so there is still time to nip that expectation in the bud. I'm guessing with 5 months to go it would cost you big $$ to move the trip to another week but honestly if there is another summer week you could go, I would move it off of his birthday and spend a quiet birthday at home... maybe have a party for a few friends and some family or something like that, but maybe attaching your family vacation to his birthday is giving too much significance. If not this year then definitely going forward.
My son is on the spectrum (aspergers even though that is no longer a diagnosis) and has ADHD and anxiety so he has been in therapy for a lot of his young life. There are times when I go to his therapist too, to discuss parenting issues and sometimes it's "parenting a kid like this" and sometimes it's "parenting in general." One of the most helpful things she told me is that a lot of parents give their kids too much choice. They do this out of love, but in reality what giving kids so much choice does is that it gives them a lot of power and a lot of responsibility that they are not really developmentally ready to carry. Right now, your son has all of the power about your family vacation because you are making it about his birthday and not about family vacation. I would take that pressure off of him... if not for this trip, then for next year. And I would talk to him about it too (most kids are pretty smart) and say "daddy and I have decided that we are going to have a family vacation that is about us as a family and have your birthday celebration separately." And I would not give him veto power over your vacation. I would maybe at some point sit down and make a list of places that your family wants to go together and then you and dad sit down and figure out where next year's family vacation is going to be. Then once that is decided give him some choices within that vacation... "we are going to new york next year. Do you want to see the lion king or do you want to see Aladdin?" I think this advice was given to me when he was ABOUT your son's age, maybe a little younger and it has made the biggest impact in the way I deal with him to be honest. Acting "bratty" or "entitled" is just a sign of discomfort with having the pressure of having to make big choices that they shouldn't be making - either directly or indirectly. Of course kids should have some choice in things but the big choices should be made by you and your husband (which you did in this case, but he is just acting like he is an equal partner in the decision). It's just a lot of pressure for a little dude (one that many kids act like they want, but what they want and what they need are often two different things... the other big piece of useful parenting advice I have gotten) to have his birthday tied to a big event like that. It's nice once in a while (as you saw by having such a magical trip last year) but I would separate them out going forward.
That said, whenever you go, he is going to have a great time. I have read tons of reports by parents about how their kids don't get excited about trips. For a lot of kids, travel can cause anxiety (as someone pointed out above it was about getting there not being there that made her kids not be excited about going). I don't know where you live but maybe NY is driveable while florida requires flying? My little guy gets anxious about flying. even if it's nothing "new" or "different" he'll still have a lot of fun and so will you and your husband. Raising kids is hard and then they break your heart

They are little narcissists in adorable clothing, lol. I think this is a biological necessity for the young of any species, the only way you can survive is putting yourself first. As thinking, feeling human beings we try to teach our kids otherwise but they will grow out of it. Enjoy your trip, I'm sure it will be awesome.