Hi and welcome. First of all eliminate all other causes for the fatigue. As others has said, he very well may be just worn out.
If it is depression, then treatment is available. My hubby suffered from severe depression for 8 months. Couldn't get out of the bed or if he did, then he'd just sit in his chair.
He tried a number of drugs, see profile, but now has found an anti depressant that works and he's a different person, more like his old self. Sometimes you have to try different drugs in order to find one that works.
Hope he gets good treatment for depression. Good Luck & God Bless
wendyr