maybe your sons hearing is different in a two person one on one environment rather than kids screaming everywhere, maybe he didnt show sensitivity in the hearing test because it was already a quiet environment and his anxiety was low at that point, he wasnt afraid or on edge when tested. but put those on him in a loud environment and he would most likely have a different reaction to even the slightest noise.
How is he on vacuum cleaners and hair dryers? Some kids say it's the noise when really it's the stress. If he thinks it is the noise, you could put him into those soft foam ear plugs when he goes to something like a moon bounce, to see if that works for him. You might find that the issue is more stress related than noise related, and then you can work on it from that angle.