Motocross is where people race round tracks against each other, trials is basically the same as push-bike trials, throwing bikes at rocks. TY80s are OK, but I'd recommend something more modern like a Beta rev and go 50, I once knew a bloke who got his daughter one. She used to beat me in competitions. Nuff said! (I was a 5ft tall 14 year old on a TY250 monoshock though, I could barely start the bloody thing let alone balance on top of it!) EDIT: Oh, and I'm pretty sure he can ride upto a 125cc as long as he's under 16? EDIT2: I'd get him a Beta 80 if I were you, I vaguely new a young lad who had one, he beat me at every single trial I saw him at. (He was also one of the top youth D riders in the country though). Try asking on trialscentral.com, you'll get more/better advice.