I don't think you have to worry about a lack of self-control later on. Take into account that the other children in class are also getting work that engages them pretty much the entire time, and that this is not an accident. No one is questioning whether they will develop normal levels of self-control later. The other children are not being expected to wait quietly for long periods with nothing to do - why should you be expected to ask this of your child?
From what you say, as long as he has access to something appropriate to do, his is well-behaved. Even if he never stops needing more stimulation than average (which is likely), there are plenty of adults who keep a book, sketchpad, or other quiet diversion with them when they know that they are going to be in a situation where they have to wait quietly for a long period, and this is not seen as inappropriate.
I realize the district as a whole is not being supportive (one of the reasons we went to homeschooling), but could the gifted teacher perhaps help you out by coordinating with the classroom teacher and providing projects or additional work for your son to do in his regular class after his regular work is done that extends either the breadth or depth of the instruction that is taking place in class, or that helps your son develop an area of particular interest?
