If he's already making a buck busking, maybe he's practising enough!

I can't see any problem at all here. It's not reasonable to expect a ten-year-old to be committed enough to practice without encouragement, although a very few will do so. Your son clearly has aptitude and seems willing to learn.

If possible try to get him into a set routine of praticing at a set time each day. Two short sessions is better than one longer one. Two sessions of 15 minutes would be great, but if he won't go with that, then suggest ten minutes each, or even five. With a very short session, it is likely he will become longer, as he gets absorbed in what he's doing. That's a million times better than having to do a long session, and losing interest before the end. The more frequent the better, obviously,but kids progress even if they practise only two or three times between lessons.

If you could understand music enough to get a clear fix from the teacher on what he needs to be working on, and could then encourage him in that direction when practising, that would help tremendously. Again, many kids progress well without this level of support.

The bottom line is, your son is an entirely normal 10-year-old (apart from the fact that he can earn money from busking).