This is something that happens with me too. I don't know exactly what it is myself - it isn't something that is usually mentioned in criteria for ASD/AS, but it is the most disabling thing that happens to me.
When I was a teenager (maybe not before, but I am not sure) I was extremely shy, and part of it was shyness, but it is far more than that because when I am feeling really depressed I can't speak even to people that I know well. To me it seems to be an energy thing - when I feel depressed, speaking requires a huge amount of energy (even more than doing some exercise) and it just becomes too difficult to even try. I do find writing easier, so maybe you could ask him to write his replies down instead. However, even with writing I find my responses are shorter and terser.
For years, my inability to speak at times made me really unhappy - it escalated things, so I would be feeling slightly unhappy and unable to speak, which would make me feel worse and more unable to speak etc. I would see everyone else carrying on conversations without obvious effort or discomfort and it would make me very unhappy. Now, finally, after over 30 years, I have stopped caring and it no longer hurts me. But this is a position that I have only reached after a lot of suffering.
I think the most important thing, is that when your son doesn't speak, don't exclude him and start to ignore him, but don't force him to speak either - you say he makes noises and I have often found that physically I can't speak properly and my voice doesn't even work, which embarasses me even more. Maybe try written communication, but this may not work. Try to speak to him as though you are having a conversation, but don't expect replies - if you need replies, then yes and no replies are a lot easier to make than anything complicated (I often give up and say nothing if more than a short word is needed), although often when I cannot speak, making decisions is also difficult. Leave him alone when he needs to be, but don't ignore him - being ignored was always the worst thing for me. Just try to treat him like normal. If it goes on for a long time there may be a problem, but for me it is usually a fairly short term thing. When I lived in France, I spent the first couple of years unable to speak a word of French to any of my coworkers (even though I could speak it to strangers and to myself fairly well). I still don't know why, and in the end it is best to try and accept it (and to get him to accept it) than to try and force things or blame him for it.