Sally 44 I have much the same situation as your son.
I have basically two emotional states.
1. Normal
2. Sad
There is a third state that occurs occasionally and that is REALLY VERY ANGRY.
Psych says that really very angry becomes obvious to me after a long build up of anger that I am not aware of, makes sense?
It all relates to alexithemia (inability to read self feeling) I was told.
I mostly don't know what my expression looks like and have been shocked by a number of photographs. One particularly I remember getting good news and there was a photograph from that moment and it looked as though I had a migraine or was about to murder someone. If you had asked me before showing the photo I would have said that because of the good news that I had a Cheshire Cat grin.