I would definitely look at getting a 90cc for your nephew (I wish I had a rich Uncle)

I bought a 90cc (E-ton) for my son last summer. He's only 9. I thought this quad would last him a long time. It turns out that he can out-ride that quad already. He is not big enough to move to a larger frame, yet (he's already trying to talk me into a Blaster!) For now I might have to modify what he has. I don't want him going faster on the flats... but I think he does need more power on SOME of the hills. If he doesn't ride safely, then he will loose mods (and we'll both have he!! to pay with his mom!)

I wish I had purchased something that could have been modified more easily. Other than that, I have no regrets. If you can get him into a safety class, I think that would be good. It's better for him to hear the Safety message from other people besides yourself. I think it helped my son approach it more seriously.