Hoarding

Maybe call a professional to help him clear his stuff.
He may then take his problem more serious.
1800 got junk?

Hoarding is a sign of mental illness.

Since you are divorced, I take it you have a separate home? Then you don't have to deal with his junk. If you are still separating out your possessions, get what you need and leave. If you have kids who have to visit him, tell him they can't come there because the hoarded stuff is a fire hazard (it is if there are big piles of it - also, often, big piles of stuff can be homes for vermin). You don't mind sharing custody but the kids have to visit at a hotel. Pay if you have to.

Since you are trying to remain on somewhat friendly terms, continue to suggest therapy and see if there is anything you can do, but it is ultimately not your problem. Calling a professional organizer is one possibility. Also, if he is complaining about a lack of money, some of his stuff might be saleable on eBay or at least could be donated to charity for the tax deduction. That could be another way to get some things cleaned out, as a play to a need for money.

But ultimately, and particularly if you do not have kids who have to visit him, it is not your problem anymore. If you want to maintain a connection, you can have lunch out somewhere and never have to see his stuff again.