Feral Cat Question

Mushi. He is a 'friendly feral' born in my outdoor space. Five years ago, just before a serious snow storm was predicted to happen, we (my husband and I) trapped Mushi's mother and two siblings. The family were living under a table and would certainly have perished when the storm came. I took the family to a vet, had them neutered/spayed then found homes for the mom and two younger siblings. But Mushi was too wild no one wanted him; so after neutering we let him go; but he kept coming back for food. Eventually he stayed, becoming a sort-of house cat...although my husband lets him out at night. We live in the back apt. on the ground floor and our outdoor space is connected to several backyards on the block with no way for him to get to the street. Mushi does not go out during the day because he is an accomplished predator and would bring home dead or dying birds....Anyway, Mushi has accepted all of my rescued dogs and even a few elderly cats; no problem. Until this past Saturday when I took in a 15 year old female cat named Tiffany whose person had died and left 4 cats. The rescue group on Long Island took the cats and placed 3 of them but no one wanted the 15 year old so I offered to foster her. Tiffany and Mushi had a hissing/growling face-off and then an actual fight, she being the seeming aggressor. I intervened in time and no one seemed to be physically hurt. After this I naturally separated them with a baby gate since I have no doors in my two room apt. . Mushi in the back room, Tiffany in the front. After this, Mushi withdrew and stopped eating. He curled into a ball and slept, refusing all food. He is usually a voracious eater. We began to worry. So I asked the rescue group to please reclaim Tiffany, which broke my heart as I had fallen in love with her sweetness. Mushi, being somewhat feral is not affectionate and even runs away from me (since I had to be the one to catch him to apply anti flea medicine); but I still love him.
So after Tiffany was picked up on Monday, I washed everything in the apt. that she had touched or lain upon so he would know she was gone. He was still in a seeming depression, not eating or drinking; only sleeping. On Wednesday, he is walking around and even drank some water, then asked to go outside. He came back within 15 minutes and went right back to his bed, rolled into a ball and fell asleep. Still not eating. I've approached him frequently, massaging his neck, which he allowed, but then he'd get up and run away. Now it's Friday and though he has eaten a little bit of food and drank some water, he is still curled up in a ball in a corner.
Does this sound like depression? Or do you think there could be something else going on? My husband has a good relationship with Mushi and is resistant to forcing him in a carrier and bringing him to be examined. He feels Mushi would never again trust him; and even feels Mushi may die. I think Mushi has an infection and is shutting down. My husband doesn't agree. He thinks Mushi is depressed. He is now lying by the entrance door to the apt. I have several pet beds and blankets, but he lying on the floor.
Thank you so much for considering offering your opinion.
Much Peace,
Sharon