I think mathyone’s argument is that he can take easy courses where he already knows the material and will be bored (and few things are more boring than to be re-taught a foreign language you already know), or he can take different easy courses and not be bored.
Does his school not offer fun electives? Cooking, woodworking, ceramics, photography, sewing?
If he really would prefer to sit through a French 2 class when he’s already fluent, then okay. But a kid who is really interested in learning for its own sake (and it sounds like your kid is) would rather learn something new.
On another note, this may be way off base, but it’s possible that your kid may be in for a rude awakening at some point. There’s a difference between a kid who is high-achieving in school because of natural ability and one who gets there through hard work. Has your kid ever had an experience where he really couldn’t get something done easily and he had to work really hard at it to accomplish it? If not, then the first time he comes up against that experience (and it might not be in the course of academics, but could be in another area of life) he won’t have the skills to deal with that. I am NOT saying that you should make him take history or another class he doesn’t want just because it will be hard for him – I am saying that you should pay attention to how he responds when asked to put in a lot of hard, uninteresting work for a goal he really wants to achieve (not for a goal he doesn’t care about, like history). If he doesn’t want to or can’t deal with the work and ends up abandoning the goal, that’s not good.