My DD is super-skinny, but she doesn't do thrill rides. FWIW, though, I have found that you need to watch out for things with very smooth seats as well, even if they are not thrill rides. Pooh comes to mind. When that ride stops suddenly, she goes sliding. Things like Gran Fiesta Tour that have the friction strip are fine, but smooth seats are her kryptonite. The best advice I can give, Lori, is to try to "teach" your DS how to brace - against the restraint, with his feet against the floor - and practice on a few mid-level rides before you do the crazier rides.
Also, I agree that if WDW has just one bar for the whole ride vehicle, that means that no harm should come to your son. I can't think that they are not aware that "fluffier" (that's my term of choice) guests ride with slimmer ones, and in rides where that is at all an issue they would do separate restraints.
Definitely ly practice or explain bracing and holding techniques. I (135lbs) rode with my thin four yr old son and he almost slid off the seat and under the bar on BTMRR. Luckily I had my arm around behind him and could grab him and tuck him into my side. His feet didn't touch the floor so he had nothing to brace against and didn't understand that he needed to hold onto the bar instead of my hand.