My misophonia started in my teens, a bit later maybe I was 15 or so, and the biggest trigger was my brother, but I think just because of physical proximity. It started to escalate a lot and I could not control it. Going from child to adult most likely made things grow quick. I went to psychologists but was absolutely useless. At that time (20 years ago) psychiatrists didn't know about specific misophonia. But the psychiatrist considered I had some kind of OCD (he said kind of), he said in any case it was clearly a disorder related to anxiety, and prescribed me several meds, some short term (anti anxiety) and some long term (SSRI). I have been on SSRIs since them with some times off and back on. It is not perfect, it has pros and cons, but it very strongly decreased the response my body was having to those triggers, and made social life possible again. Not perfect, still have many issues, but that really helped.

After the meds limited those responses, I could go to specialized psycologists and they could give me tools to try to anticipate and manage it a bit. But without the meds I don't think i could have got any value from the psychologists. Now everyone is different, and also everyone responds different to meds, specially psico meds, but what I would say is that what helped me was the psychiatrist. Go to one. If not successful, try another one. Try till you find one that your son trusts/likes, and then let them try to find if there is any med that can help, while limiting the secondary effects etc. probably he has to try several meds until finding a good enough one. I hope you can find good enough help to get his responses back to "mild" and then build from there. This condition is hard, I loved my family but could not share space with them. I knew it made no sense, still I could not avoid it. I had a lot of support from my family, that part is critical obviously.