You wont go far wrong with any of the maverick range tbh, and for the little bit more you can get a 1/10th scale truck or buggy that will go anywhere.
As good as the Associated 1/18th is it is an 18th and your lad will be limited on where he can run it
Check the maverick range out here
http://www.hpiracing.co.uk/catalogue...and=MV&pg=CEPF