Maybe to answer your question directly, from your description, I'd recommend you look at the 2.4 Ardent for the front, and something like the crossmark for the rear. It'll take a while for your son to find the tyres he likes though. It's quite a personal thing and depends on riding style, terrain and weight. Edit: Just to add, the wife rides Schwalbe 2.25 Rocket Rons Front and Back, and loves them, but she also weighs like 49kg's and isn't an aggressive rider by any means.. So ya.