I think you just have to focus on your own happiness and then you will see that it creates ripples of happiness around you as well. In an SR model, he would be mirroring some disappointment you have in your own life. This article of Steve's explains this perspective:
Understanding Human Relationships
Even if it's not "true", it's probably the best outlook you can take on. You have zero power to solve his disappointment, but 100% power to solve your own.
PS - Also try not to do or say anything to reinforce his disappointment. Never tell him that his job sucks or he is unhappy for example. Best not to tell others either. Words have power. If you want to make observations about him, try using the strategy of
Negative affirmations and state things as "not-positive", such as "he is not happy" or "he doesn't have a fulfilling job" or "we don't get outdoors enough".