Not tired. Unless he can barely drag himself out of bed a man will find the energy to have sex. I've done it with fevers over 100% and after being awake for 90 hours. Men will pass out trying to have sex before they give up.
One of two things are likely happening here:
When people are clinically depressed they often describe a reluctance to engage themselves as exhaustion. In those terms it feels to them as though they don't have the energy to commit to the act. If there are other things he's too tired to manage, social engagements, trips out of the house. If remains in the house all day even when he has nothing else to do you may want to talk to him about the possibility of seeing a doctor about what's going on.
When young men are having a problem sexually they will do just about anything to deflect discussion about it. It could be performance anxiety or a lack of interest to the sex you two have been having or infidelity.
This is much more about the fact that he's not being honest with you or himself than you not getting any action. If you want to keep this relationship you're going to have to confront him about this. Tell him you need sex with the frequency you used to have it, that you're willing to discuss what you can do the help make that happen. Emphasize that you want your relationship with him to work. This is situation that will get worse if left alone.
