Yes, I'd be very surprised if they didn't catch something more serious. The back pain should improve as his gait and running stride normalize, which PT can help with.
We had a sledding accident. Im pretty sure they did a full trauma work up at the hospital. All sorts of X-rays. Couldnt they have seen something then? It all started after he started trying to run and stuff, which is why it makes me think its muscular imbalance?