I feel for you. Lack of sleep, physical exhaustion, and mental stress are each powerful enough on their own to trigger depression... he appears to be dealing with all three at once. I can totally relate to that with my own partner, those same things derailed her big time.

Have you tried talking to him about seeing a doctor? Going to therapy? Trying medication? etc? Having experienced your own depression, you can maybe frame the conversation in that regard? Does he have depression in his family? Parents, siblings, etc? Do you know if, when he sleeps, he sleeps well and regular?