If his ankle doesn't look too bad to you, I would recommend the following: Keep your son's ankle elevated, putting ice on it. Give him some ibuprofen (Motrin) or acetaminophen (Tylenol) for any pain and swelling. Limit his activity, and have him stay off his ankle for a bit. Although injuries take time to heal, many will quickly improve with these conservative measures. As he seems to improve, have him stretch his ankle gently and gradually increase his activity level. If after five to seven days, he's not getting better and especially if he seems worse, contact his doctor.
Team sports are a wonderful outlet for children and I always encourage active participation with good sportsmanship. Injuries, unfortunately, do occur, but children are more resilient than adults -- they tend to bounce back and recover quickly.