He should see a Dr about his tiredness.
One thing that can cause this, especially mental tiredness, is disturbed sleep. If he is stressed, lacking self confidence, woried, these can all contribute to disturbed sleep.
But one thing he does need to do, and by he, I mean him, not you continually telling him, is get himself into a routine. getting up at the sort of time he would for work and going to bed at a reasonable time. Also, he need to get at least 8 hours sleep a night, so the arithmetic should be simple.
But he does need to see a Dr about his tiredness.