Get him to talk to a counsellor - a counsellor rather than a psychiatrist who will likely prescribe expensive, unnecessary, and potentially harmful drugs. It could be a mid-life crisis he's going through, it could be some deep-seated emotional problems, it could be something psycho-somatic; you don't really know. What's important is that he is given an outlet to talk about whatever is bothering him. A neutral party is often the best outlet, not family members. What you can do is offer emotional support, and yes, do things with him, open doors for him to come out of his shell.