Sounds like it could be his stress, age and meds. It's understandable if he's getting frustrated w/himself. He's thinking he's not a spring chicken any longer and yes, he can see the changes in himself and his performance and that could be depressing and scary for him.
It's not you...it's him and he's going to have to figure this one out. If he should talk to you about it, gently suggest seeing his doctor and discussing the issue w/him. He may not feel comfortable in doing so, but it's a normal problem and trust me, there are advertisements all over the TV these days about it and what medicine is available to help men, etc.
