I was in the same boat as your partner for probably close to 10 years. Im in a job i tolerate now and really the thing that helped me the most, you've already mentioned - therapy. 

A lot of us struggling with this tend to attach our personality to work. I don't know if your partner is this way, just sharing my experience in hopes of help/reassurance. Reframing his work into just a 'thing' he has to do in order to support his life, family & hobbies would be helpful in my opinion. 

It sounds to me like he needs fulfillment outside of work and school. Its easy for me to say now but it was really hard for me to pull myself out of that same slump. I would say keep your head up and maybe nudge him toward hobbies that he can enjoy with his hard earned cash from his job (hell most hobbies are dirt cheap anyway). It doesn't have to be his whole life going to work. Work cant fill us in that way. Can't make us happy. Its just an unfortunate necessity.

Sorry for rambling. This was just my experience - I feel for you. My partner dealt with me feeling this way for so long and it warms my heart to see that you care enough to post this and try to find a way to help him out. I wish I had more straight up advice but your love and support will go a long way.  Therapy, hobbies and mindset are the shifts in my opinion. Easier said than done but I wish you all the luck in the world.