In a way, your son is way ahead of his peers because he is aware that taking something from others doesn't make them happy. He gets empathy.
I would look at how to help him keep his empathy and still get his rights kept. Besides the martial arts classes, i would work with him on learning how to negotiate or problem solve instead of a black and white 'I have it vs. you have it". This way you aren't working against his basic nature but rather adding skills to it.
Often when the kids in our neighborhood have these scrabbles, I do a "Billy wants this and Sam wants this too. What can we do?" then let them think, offer suggestions of trades or alternates and it works out 99% of the time within 3 minutes. My son is learning to approach things this same way and it is amazing to see.
I got started doing this with something called Non-Violent Communication by Marshall Rosenberg. My imperfect translation of his basic theory is to identify what each person wants (and have both state the others need) then a solution is a few minutes away. I got an audio book of his out from the library.
It needs to be translated for kids, but I don't know offhand of a good resource. maybe search on the web for nonviolent communication for kids, or even search for a NVC (nonviolent communication) group in your area. There are a surprising number of them out there.
Hope that gives you some ideas.