Did something happen a couple years ago, did he lose his job or you all have some sort of setback? I would try to approach him from a friendly/loving angle, one of concern when you recognize he seems "absent"/more Cloud 1-4 instead of 6-9 :-) and just gently talk to him about how you feel, how it hurts you to see him unhappy, is there anything you can do for him, how you are thinking of seeing someone to help yourself/your own problems and maybe he might like to go along, see someone together, etc. Just get a conversation about "life" going that cannot be viewed as about him only or blaming, etc.