From what I understand of dreams, we use dreams to work out the issues our waking state can't/won't so that we can have balance in our waking state - but that is just one theory.
There is no definitive answer to what a dream is or why we have them. Let alone why some of us remember them so vividly and some of us not at all.
I don't think its uncommon your husband can't remember his dreams. I usually only remember really intense dreams otherwise my dreams to slip away quite quickly and I can't remember the details or specifics, just the fact I did dream.
If your husband can't remember his dreams, maybe you could record the things you hear when he's dreaming and look up and try help him interpret things he may be feeling/dreaming about and give you both some clarity?