Spacious is relative. I've been in many restaurants that have high ceilings and open concepts but the tables are all crowded very closely together. I suspect you're asking for places where the individual tables don't feel surrounded too closely both others.
We felt that our table at Coral Reef didn't feel as "crowded in" among the other tables as we did at many other restaurants. If your party is big enough and can get a booth that would be even better. Just make sure to clearly state that you do NOT want to be up against the tank because for some reason people seem to think that it's ok to crowd around other people's tables if the table is against the tank. It's a request they can easily grant since most people ask to be at the tank. Even if you can't get a booth, being in a table in the second or third tier just feels more open. The place is dimly lit though (it's necessary in order to see into the tank easily) so keep that in mind.
At 1900 Park Fare, if he can handle characters, there's an annex area that most people DON'T want because it feels out of the way and almost hidden from the rest of the room. This might be perfect for you. The characters do come in the area and you get just as much interaction as everybody else but the area's just more secluded and out of the way. The tables in there are still kind of crammed together but at least there aren't a lot of tables in there so it's manageable. You do still have to go up to the buffet to get your food in the main room.
Garden Grill is wonderful. We sat on the lower floor so I don't know what the upper is like. On the lower level, every table is a secluded booth with high backs so you don't see the other diners. You also get great interaction from the characters but it's low key interaction. It's all you can eat but it's brought out by your waiter so you don't have to get up. IMO this is the perfect environment as long as he's ok with the changing scenery (the restaurant rotates very slowly). Depending on how high functioning he is, he may even be fascinated to ride Living With the Land to see where the veggies and fish that he will be served (or was served if you go after dinner) are grown. That ride is a great sensory break from the activity of the park and may help calm him down before eating if he's getting over stimulated.