I agree with the other two comments. Have him see a counselor. Also, just by looking at your post, I can see that he is the middle child and has a very young brother. He is probably still adjusting to not being the youngest in the family, while at the same time beginning to go through the "changes" life brings at the age of 12.
It is great that he is involved in a martial art, but I would try to provide more activities that promote engaging with others. Martial arts is more of an independent sport and usually has several different age groups in one class. Team sports (where kids are the same age) provide so many positive results. With him being home-schooled when does he have time to socialize with his peers? Just a few thoughts to ponder.