He isn't depressed he told you so why don't you believe him?
You are failing to grasp that there is more than one way to escape depression.
A prime example of this is in those that recover from strokes.

I think I know where he is

He has chosen is to revert to an earlier option and this choice has aborted all of his defenses learnt between the earlier option and where he should be now.

He has to survive on this because he feels there isn't anything else available to him.
He does however have a problem with his illnesses and is anxious about them.
If he could make contact with them to find the solution he would but he's fearful
for to do so he would have to switch off the earlier option and switch the unknown back on which is depression.

He has found a solution that though not perfect (and all his family can see it as such) which will develop in time with encouragement to where he needs to be to recover his defenses and part of his defences include yourself.

He's sitting at home with you, as if you are his parents, feeling trapped and at the same time being obnoxious without daring to risk the brave new world.

His bond to the old ways and to the parental way (even if they are not alive) is greater than his love for you because the only love he knows is the maintenance of himself.

In the same way as parents resist maturity as unreadiness, the partner resists it.
Maturity is the integration of body state with the will to navigate away and integrate the body state into someone else.
If the body state isn't felt to be perfect he will stay as he is
If he is told these afflictions are commonplace he might venture out