B.H.
I'm no expert but my first instinct would be to enroll him in some sort of individual sport like swimming, karate or something where the competitive aspect is taken out... kids can be rotten when picking teams and ...even if picked, it's no secret how bummed they are when one kicks or hits the ball poorly... doing something independent where he isn't competing with others could be the confidence and strength builder he needs to set him up for junior high and high school where team sports are more of a thing...
even then, he may choose track and field or something that isn't team oriented... the important thing is that he does get that exercise and confidence!
good luck!
good luck!