If I try to come onto to him he is always "tired" or "not in the mood." I am unable to excite him most of the time and the few times I am able to excite him he still says he isn't in the mood even though physically he obviously is.

Has he had a physical lately? Does he have problems at work, financial difficulties, something else that is "heavy" on his mind?

That's the thing I wrote him a long letter about the whole situation and talked to him about it after he had time to let my concerns as presented in my letter sit in. We have been in a stressful transition however these sexual issues have been going on for over a year. I hint to him about getting a physical but he always says he is 25 and doesn't need one for "those" reasons.