It seems nowadays that a big proportion of guys (more than 50% in the UK) admit they are often too tired to have sex after a days' work.
Stress just kills stamina and libido. A guy might need to recover energy and sex requires time and energy to be good.
Another common issue is timing and space. Guys sometimes feel pressured at the wrong moment (for instance early morning before taking off to work)
Don't pressure him.
This will make it worse.
Invite him instead.
Let it go if he does not respond.
Most of the times, it's not a "therapy" issue.
It's more related with life style.
Have open communication about your own needs.
Don't challenge him.
The best is to give him space.
Give him some form of subtle signal that it's not enough for you but give him space to take action in his own time.
The good news is that there are good chances that this has nothing to do with you, rather something he is going through.
It would happen with anyone and it might be happening to millions of totally healthy guys today.