I have misophonia, my experience is just like your son's. It started in childhood. I am also autistic (which I didn't know until last year, I'm now 35), and misophonia/sound sensitivity/other sensory sensitivities are common in autistic people. But non-autistic people can also have misophonia.
We are often 'triggered' by family members, for me it's my mother and father, but I also get it with strangers on public transport or certain colleagues at work.
Please know that misophonia is not simply a feeling of annoyance or slight discomfort at hearing the 'trigger person' eat, it's a horrible, all-consuming sensation of panic, rage, fear, adrenaline spiking, 'flight or fight' - at least it is for me and many other misophoniacs. It's extremely distressing and has caused much suffering. I would not wish it on anyone. It's something we can't help, can't 'get over' and can't ignore in the moment. It's truly a horrible sensation and added to that the fact that we are being triggered by people we love, who we don't want to hurt.
I relate very strongly to what you say about your son seeking means to escape the noises. I know you say you don't eat loudly, slurp, etc. but it honestly doesn't make a difference, people with misophonia/sound sensitivity can pick up tiny noises and will hyperfocus on them, even the quietest of noises. The slightest swallow or lick of lips. And even if the noises are barely audible, sometimes just seeing the mouth move or just knowing the 'trigger person' is eating is enough to cause fast heartrate, adrenaline spikes, stress, anger, fear, etc.
I don't have much advice really - you are correct that it can't be treated and the causes are not known (except for autism, which is the cause of mine) - but all I would say is please try to be understanding. Speak openly with your son, ask him to explain how he feels and how this affects him, try to come up with ways you can make home and family life less stressful for him considering this condition.
Please message or @ me if you want any more details or advice, I have to head to bed now but happy to speak more about this/my experience/my thoughts.