If his anxiety increases, he should not be back at school. He should be able to remain symptom free, especially the anxiety sensitivity and nausea, before returning to school. The anxiety is cause by his brain being overwhelmed with stimuli. The sights and sounds are too much for his brain to process.

Has he tried wearing foam ear plugs to reduce the auditory stimuli? They might be just enough to prevent the relapses.

You should even watch for delayed symptoms. If he makes it through a day at school but feels poorly the next day, that is likely a delayed symptom of the previous day at school.

My best to you two.