Amtali, I would be very disappointed in my children if all they wanted to do was study, and didn't have interests and hobbies and social activities that make them a well-rounded person.
Music is actually good for math skills, btw - studies show that students who practice and are involved in music do better in math than those who don't play music. This is such a growth time for him - 1 1/2 years of Judo is a good commitment, now he wants to do something else - all of his interests are admirable and healthy that you've listed. I would encourage ALL of them, even if he tries them and decides they aren't what he wants to continue with. It's a healthy child who continues to want to try new things and experiment with different activities. All work and no play creates a very dull personality and leads to a dull, dull life.
Try not to worry too much...this sounds like typical behaviour for an adolescent of your son's age. It sounds like he is just trying to figure out what his interests are. This is a good time to help him learn that he needs to stick with the choices he makes and he needs to honor his commitments. The plus side is is that it sounds like he is a well rounded kid...goodluck!