take him to a real MMA gym that takes kids classes or a BJJ gym so he can work on his top and bottom game. At home go onto youtube and watch BJJ instructional vids. The 10th Planet videos are really good.
For about 2 years I practised BJJ at home with friends (I'm 155, my training partner at the time was over 220lbs). When I finally got to a proper gym my jiujitsu was really good! I took a purple belt's back and almost choked him out but I was too close to the cage so I couldn't roll over and get a body triangle and he eventually turned into guard.
2 years of wannabe BJJ + 1 year of MMA training (I only ever went to 2 NoGi classes over that whole 12 months) and I was equivalent to blue belt level. I'm not a super athlete, i'm just a flexible guy that has a lot of interest in the sport, therefore I can practice the techniques that I see without my body giving out and as it's one of my favourite things on the planet, my mind soaks it up.
Sorry for rambling but the gist of it is: Do not underestimate youtube BJJ vids!