Maybe he has ADD and just gets distracted?
I'd probably just raise the subject if it were me. In fact, not specifically body language, but due to my husbands difficult childhood, he missed out on learning quite a lot of the normal social cues most people pick up naturally. They're mostly the sort of cues that occur in a relationship between two people so we regularly talk about stuff that he just doesn't get. Not so much these days, but earlier on in our relationship.
Anyway, that's my 1.63 cents worth too.