Alex, I'd make him an appointment immediately to be evaluated by an orthopedist, preferrable one who deals in sports medicine. I'd also talk to the coach. When my own son had an injury at the start of the season, he wanted to come back the day he got off crutches. Coach said absolutely not! He has the authority to protect your son. For some reason those teenagers don't think us parents know what we're talking about , but if the coach won't play him until he gets checked, player has no choice but to wait.
Good luck and hopefully it's nothing. He doesn't want to risk it - get evaluation!