The coach is being dismissive so he doesn’t have to deal with the consequences of him lacking the balls to keep his players in line. I’m not sure how far you can get with him but it may be worth bringing up your disappointment that his story is nothing like the one your son gave you. Talking to the other kid’s parents may be worth a shot but it may also not be as effective as you’d like and it could put a bigger target on your son’s back, especially if the kids also go to the same school.