My goodness, he has been through so much!

Have you noticed if this is a seasonal thing? This time of year, you keep windows closed, the heat is on, and the indoor air gets dryer than it is at other times of the year.

Have you tried giving him a little canned food daily? Not so much taht he gains weight, but just to increase his fluid intake? Sometimes that works.

And you can add some oil to his food and see if that helps. A little olive oil, drizzle it over his dry kibble. Maybe about one tablespoon per day. Not a lot. It won't harm him, and it may give him more moisture.

Those are the only things I can think of right now. Good luck!
Sandra