He /she has a nasty matt by the looks of it - I would guess that is what the lump is. Have you tried getting your fingers behind it to see whether it is just held on by hair? The only other possibility would be a tumour but my money is on a simple matt.

If you can take him to the vet, they will be able to scan for a microchip and will not charge you for doing that (phone first to check though!) - that way you will know. But if that matt has been there for a while it does seem as if the poor thing is likely to be a stray. Still, his owners might be desperately looking for him.

Don't hand him over to the RSPCA, in some locations they are very keen indeed to destroy animals (where are you?). If you need to find him a home, try Cats Protection instead.

Liz