I've got this problem with my old cat, it does hurt them when you try and sort it. The vet has to lightly sedate her and shave her. Sorry this is absolutely no help to you, but cats are too wriggly to go near with scissors or shears.
When we had an old long-haired tomcat who was overweight we occasionally had the same problem. I managed to cut some of the knots out carefully with scissors but it was a two-person job - my husband held him.
If he is overweight he probably has difficulty curling round to groom certain parts.
If I were you I would have a polite word with the people you think may be feeding him. I have a real thing about strangers feeding pets "on the sly". If they know he is yours and he is well cared for, they shouldn't be feeding him.
They just did it as part of a normal consultation, so about £21 a go but it is only occasionally. We brush her in between times to prevent it, but her health condition makes it go worse quickly sometimes (iykwim). She hates the vet too.