He may well be tired, unless there is evidence otherwise. How is his stress level? How is his work load? How is his sleep? There are a number of things that can tire one out. His lack of sex drive could be that your relationship has fallen into a pattern. Sometimes, minor changes can spice up a relationship when two people have lived together for some time. When you have sex, is it done in a similar fashion each time? Does he have certain interests that are not being met? Does he know how you feel?
i would be suspicious, he could be cheating or he could watching porn and masturbation. i want to point out that the obvious age difference will affect your sex life as well.