As I understand it--and I'm not an expert--they're pretty rigid about not letting kids under 10 or so move up from Cub Scouts to Boy Scouts, so I'd probably recommend that you go more with his age rather than his grade for this one. Since most of the stuff they do isn't highly academic, this seems to be working out fine for our DS7 this year (in a den with several GT kids). The fine-motor skill stuff tends to be more important for Cub Scouts, so I think it might be better if you go more with the age-appropriate peers rather than the intellectual peers. And there's LOTS of freedom of choice of what to do, so even if some of it is too easy for him, he doesn't have to do it (or he can just get lots of awards easily!).
At Pack meetings once a month or so, he will associate with all the kids in the pack, so it isn't like he'll never see older kids.
I guess I think maybe put him in Tigers? Preferably with a pack not in his school, I think, since then he's with the younger kids at Scouts and the older ones at school. It doesn't sound like an easy fit there...
Worst case scenario is that he has to repeat Webelos because he's too young to move up. If that's going to be a problem, then you might want to wait another year.
You might also talk to your council. Perhaps they can make an exception in your son's case? Our Pack Leader made it sound like that wasn't ever going to happen, but that doesn't mean it's so!
