Tough call.
Ultimately, your son might have a good time but, on the other hand, there presumably is a risk that he might not. And even if he does have a good time, he might take some "ribbing" from his friends in any event...in fact, is it possible that that is what is at the heart of it all?...Do you really think that DS would want to be caught in a picture with the rest of the family standing in front of Cinderella's Castle with silly grins and wearing Mickey Mouse ears...you know, the one that you will get in 8x10 size and hang prominently in the livingroom? Anyways, pre-trip planning and getting your family worked up in a frenzy to want to go is part of the fun for most of the people that visit these boards; much of that excitment will be missed if you are spending a good part of the pre-trip time trying to convince somebody that doesn't want to go how much fun it will be and that, notwithstanding what their feelings are, they're coming anyways.
Would you be able to keep your itinerary flexible during your time down at WDW? If so (and if you are going to have a car), perhaps you can sell DS on the idea that he and DH can go do other things while you and DD spend your time at the WDW parks, provided that DS spends at least the first day with the rest of the family touring one of the WDW parks...even if DS has a great time with the family, he may want to go to other non-WDW attractions just so that he can tell his friends that he wasn't stuck at WDW with the rest of the family the whole time. Anyways, there should be lots of other "cool" stuff that DS and DH can do in Orlando. Apart from the other amusement parks (Busch Gardens, Islands of Adventure, Universal Studios)...how about sporting events...such as an NFL football or NHL hockey game in Tampa?...Kennedy Space Centre...golfing...
Good luck with your planning.