He's just being cautious. The kid is very timid, but well liked. My Bro and I grew up in shitty areas where we had to fight all of the time, now he has money and lives in suburbia where the only threat is boredom.
If he is timid and well liked, I Think sports might be better. No one knows how to handle a situation unless it happens. Hd could take lessons and cave like Flex Wheeler did in fights as a youth. He could go in for lessons and get handled by a girl---and that always blows. Or as the saying goes, those who are bullied become the bully.
Don't know you or your bro so I can only give those examples. If the kid likes the martial arts then go with it, if he hates it, it will show. Team sports as I said is probably the best way to go. You have the comraderie, the learning, the attention (and the pressure) and both dad and son can follow the college and pro level together.
Then there is always model rockets---the kid who knows how to build bombs and rockets, everybody fears.