Pulled tendons

I received a text from my son at 3:30pm asking if I could pick him up from school as his arm was in a sling. He'd injured whilst playing football ( and had carried on playing for 20 minutes after injuring his arm). On inspecting his arm, I noticed that he had no hair on it. I asked him what had happened. He'd cut them off during his mandarin GCSE this morning because he was bored.
At 5pm, he was in quite a bit of pain so I dropped him off at Teddington hospital (daughter #1 went with him). I then took daughter #2 to kayaking (daughter #1 didn't want to go as she has a cold and a sore arm from her injection at school yesterday).
I then went back to the hospital. He has pulled the tendons in hit arm.