The core of our happiness isn't formed due to another person. We gain happiness within ourselves but other people can supplement that.
I'm not sure how you would respond to his question of what you want from him but he seems to be trying to figure things out. Don't argue with him on what happiness is, you will never win that battle. Its something he has to figure out on his own.
If he does keep going into how he was unhappy in the M you can validate those feelings and say you recognize the issues and dysfunction that was present in the M. You saying "its not my job to make you happy" puts him on the defensive and doesn't take his feelings into account. Its not showing him you care about how he feels.
The issue here is he feels you didn't make him happy, those are his feelings. Everyone is entitled to the way they feel, they aren't wrong or right. You can validate his feelings without agreeing with them.
Validate and keep giving him space and avoid pressure, he still has things to figure out.
Accept what is, let go of what was, and have faith in what will be
