an XR400 would do the trick, but in the scheme of things it is a bit outdated compared to the wizardry of the new bikes out there, but a very good choice if your doing purely "fun" riding, trails, and just want to keep your son company. It is also absolutely bullet proof. if you break one, im impressed.
Other things to look at are TTR250 from yamaha which has electric start, which just ads to ease of riding, especially when learning clutches. the KDX200 or KDX220 from kawasaki, which have a bit more zip then your average 250 fourstroke, and can seriously haul with upgrades. I loved my KDX, and it was tough as nails, not as much as an XR400, but still put up with all my learning-to-ride abuse. I may be a bit biased, but the KDX is a great little bike because its cheap, and can grow with any rider because of its ability to seriously hustle with just a few aftermarket peices.
if your son is going to spend a while on the XR100, I think the XR400 or TTR250 (its more a matter of if u want the electric start or not, or if your brand loyal) would do just fine. But if he learns quick, or has friends that ride bigger/faster bikes, he'll be pulling on your pant leg begging for a CR85 in no time. if thats the case, he'll leave your four stroke 250 in the dust :laugh: Watch yourself out there, little kids learn quick how to do some amazing things on bikes, i love it when im struggling on a motocross track and some little 80 goes buzzing by at warp speed making it look easy. trust me, it will be no time before your son starts doing that to you. :thumb: