I've got a Slash 4x4 that my 7 year old has been driving most of the time and I am a firm believer in the "Training Mode" feature they have on them. It is fast enough that he is learning how to drive the truck but not enough that he is breaking parts and when I get to drive it is easy to turn off.

Every now and then when he complains that he wants to go faster I'll turn it on normal mode, let him flip it a few times, bounce it off a couple things and turn it back to training mode.

Another option is upgrading batteries. I'm not familiar with the layout on your truck but with the Slash we can run 7 cells or 6 cells and there is a noticable difference in the speed between the two. I know budget is a concern but maybe stepping up to LiPos would give the speed he is looking for without upgrading hardware and they can be looked at as a long term investment in the toys.