Hi TempusFunghi sorry to hear this, he sounds like a great lad and it's horrible that other kids take advantage of his good nature.
My DD started Taekwan-do a year ago, sadly, she stopped doing it but I have now taken it up myself! In the adult class of course! They do classes for younger kids, junior plus type age and adults.
The classes are local, not too expensive and the atmosphere is very good.
They run a class for younger kids which is all aimed at teaching skills like respect, confidence, fitness, how to avoid conflict, communication and coordination.
I expect karate and judo would also help to teach this kind of thing.
Hopefully too you are encouraging him etc (I am sure you are) which is all building up his self esteem etc because these kind of unpleasant incidents must be horrible for a child.
Good luck, all best wishes.