Feral cat visitor with fur problem

I've been feeding a feral cat that has been coming into my house via my
late cat's cat flap.

This small black cat is extremely cautious, however he now lets me stroke
him while he's eating, and sometimes beforehand.

He seems to have something wrong with his fur on the rear 3/4 of his body
(from his shoulders backwards, getting progressively more noticeable
towards his buttocks). The affected fur seems short and dull, and uneven in
thickness. I'm wondering if he has been chewing it, because of some kind of
irritation or something. However, I've never seen him doing so, and I've
never seen him scratching.

The cat is far too timid to be picked up and taken to a vet without causing
him fear and distress,and I don;t want to betray the trust we have
painstakingly built up. I could probably grab hold of him long enough to
apply some Spot-on flea drops to the back of his neck, but I don't know if
fleas are the problem. It could be some other kind of parasite or skin
problem, couldn't it?

Can anyone advise?

Thank you,

Al