There are crowds. There is noise. There is overexcitement for all kids, which can be even harder for those on the spectrum. I think this is what people are recommending about letting DS in on the surprise - so you can prepare him for these things. Watching planning videos, youtube videos of specific rides, looking at planning books - those all help to prepare such a child for the situation he/she will be in while at the parks. I understand the frustration with him being so excited months ahead, but does he have issues with sudden sounds, loud sounds, unexpected dips/drops on a ride, sudden lights or bright lights and dark, splashes of water, seats that feel like they wiggle, etc. These are things that most of us try to prepare our kids in advance to try and reduce likelihood of a meltdown or freakout scenario on the spot.
In an earlier post you said you went to "us" last August - I'm not sure if that is Universal Studios or a typo. If you did go to Universal - WDW will be much the same. How did you son fare on that trip? Were there things you wished you could have avoided, or prevented, or done differently if you had known? In general, many ASD kids do well with earplugs or headphones to reduce/cancel out the noise and stimulation. Also the safe haven as suggested previously. Someone mentioned handheld games or something that he'd like to play with in lines - keeps him occupied and keeps his mind off the waiting as well as focusing on the game can help shut-out some stimuli. Snacks and drinks - CS locations will give free ice water on request, we carry Mio or powdered drink packets if flavor is desired. And flexibility to change plans and return to the resort if it becomes too much for him.