You will have to be direct and firm. You will have to set limits for him, and more importantly for yourself.

Simply take his call, tell him how much time you have (your personal decsion about how much time you want to spend on the call) and when that time is up, tell him and end the call.

If you tell him you have 15 minutes to talk, then when that 15 minutes is up, just matter of factly tell him that the 15 minutes is up and you have to go now. If he can't end the call on his end, then you will just have to repeat that and say goodbye.

You will have to do this consistently for a while. He won't like it, but you have to do this for yourself and not give in to please him. If he shows up at your door immediately after, you will have to tell him through the door that you are busy and don't have time for a phone or an in person visit. He won't like this either.

He won't like these things but you are doing them for yourself.

If he can get to a place where he is receptive, you might be able to suggest that he needs someone professional to listen to him and help him with the things that concern him.