I would highly recommend doing some mild research on a condition called Osgood-Schlatters disease. I got it and it's not fun at all. Basically what happens is that if you really push yourself before your bones are fully developed and grown and hardened, basically your quad muscles can slightly rip the ligament off of your shin bone over and over, and EXTREMELY SORE knotty scar tissue will grow around this area. I am 30 and I still have a knot that is slightly sore, but it takes a bigger hit than it used to in order to cause pain. When I first got it, the least little thing that hit it had me on the ground in the fetal position holding my knee in pain. That's not exactly high-quality childhood memories right there. The memories of starting playing team football at age 7 and going to nationals in track at age 12 were awesome, but the knee pain was hell.
That being said, I see no problem in doing extremely lightweight things just to get him excited about exercise and learning properly. I would keep it on the same level or lower than what he's doing for play though. Don't go beyond that and you should be fine. Basically I would recommend only doing things to get his self-esteem up and let him feel that he's getting to hang out with his dad and be like him, even if that means only going through the motions. I would definitely NOT recommend actually trying to devise a way that he's actually packing on muscle due to the above paragraph, and there are worse things that can happen than what I mentioned.