Its excellent that you are spending time to develop you childs obvious talent.
My daughter is 7 and is learning to play the flute. She is no where near as advanced as your son, but she enjoys practicing for about 15 - 20 min a day.
We went through a short phase where she wanted to stop. She felt too much pressure while learning. Some of it was probably my fault, but she is also hard on herself. My solution was to back off, and come up with creating a daily HABIT of practicing 5 - 10 min. We've been slowly increasing our time, but the key was to keep it fun and pressure free.
At this age, unless they are driven internally - they could drop the instrument pretty quick - and all of your time (and money) for nothing. Emphasizing the habit of practice (or exercise, study..etc) is the best thing I can think of for my daughter at this age...results should come later (I hope!).
If she continues, there is plenty of time for her to develop as a great musician. How many of us newbies and comebacks are kicking ourselves that we didn't continue when we were in middle school?
This was my experience and it seems to be working out. Only you know how much you can push your son. Good luck!