At his age IMO I think pull-ups, Push-ups different variations and also if he like to run let him run.
Also something like plank hold, side plank for 15 seconds and super mans for 15 seconds also. Crunches at his age maybe even though they don't do much it would be a start.
Pm me and I might be able to give more exercises
just remember flexibility of the muscle is more important then the strength of the muscle. So have him stretch out full body before and after ANY ACTIVITY.