This community covers anxiety disorders and other mental health issues, Sophie.
The posters at Wellsphere's General Medicine community should be able to provide a more authoritative answer.
I don't think this is related to an anxiety disorder, though no doubt it is causing you some anxiety. Your family physician should be able to determine the cause and be able to suggest a remedy.
I assume your son's physical injuries have/are being treated, but it sounds as if his mental injuries aren't. These can be as damaging to his health as the physical ones, perhaps even more so.
A good starting point would
? That is a common area in the lower back that can have subluxation, causing pain, and also decreasing flexibility.... There are also ligaments in this area as well as overlying muscle which can spasm (and cause pain).
Due to the SI Joint...