I agree with map, you want something simple. If you want him to climb hills, get a larger rear spocket, your stock kit comes with a 44T with the happy time motor as well as the 49cc grubee 4 stroke motor. everyone's opinion varies but I think a
50T sprocket would be a good choice. You have more acceleration/bottom end and mid range, but top end speed would be lacking. The 49cc 4-stroke kit would be an excellent beginner's choice although keep close tabs on that gearbox, those like to start slipping after so many miles.... The HT motors are also a great choice for a beginner and they don't hurt the wallet! Replacement individual motors are pretty close to a dime a dozen(about $100 or less from reliable ebay sellers, avoid the commonly talked about no-good ebay vendors discussed on the forum here). I think if you got a HT kit for him and ran a larger rear sprocket, he'd be climbing hills and loving the thing, that way if he breaks it, it's cheap enough to fix/replace. So, to start out, I'd start as basic and cheap as possible, and once he's ready THEN level up to a more expensive rig such as the 99cc or 212cc Predator on a nicer bike. It's a leraning curve for all of us, It will surely brighten his day! Best of luck to you!