One is never to young and never too old to learn something new. Lots of dojos and mma gyms do private lessons. It wouldn't hurt to go set up a lesson for you and your son together to see if he likes, it's not going to kill him to try. A good instructor will know how to make it fun for him so he has a good time learning. Find a gym where there's other young kids too that he could socialise with if he's up to it. Personally for modern day self defence I would go MMA, Mauy thai, boxing, or just Brazilian jujitsu. That's my preference but it's up to you to do a bit of research and go with your gut instinct. Watch some YouTube videos of different styles in the ring and street. Try around multiple gyms with a private session to get a feel of the instructors/ their gym. Most importantly teach your son how to believe in himself, how to stand up for himself mentally, teach him how to avoid toxic people in life how to choose the right choices. Defending himself comes from the inside first.