Is he in his fifties? Often men go through their own mid life crisis when they turn 50 plus. Unfortunately you cannot fix his depression and you can't fix it if he is unhappy with himself either. If he is taking an antidepressant some antidepressants have sexual side effects and a man can lose sexual interest which can create some distance in a relationship. He should also have his testasterone checked as a drop in that can contribute to low energy and feeling depressed and moody.

He may not have anything going with this tutor other than he just needs someone to talk to. He may be feeling very unfulfilled, that is something HE needs help for and he should be seeing a therapist.