Usually, if it's a squishy lump, you might just have a little benign growth. However, if the mass is solid and won't move around easily, it can be a malignant tumor. Your vet can take a biopsy and prod it around to see.
This could be a cyst, or it could be tumor, benign or otherwise. If he's fightish, an injury could collect fluid- palpate it to see if it is fluid or solid, see if there is pain, and decide on treatment accordingly.
a common reasons for a cat to have a 'lump' is a cat bite abcess. so he may have been in a fight and now has an infection under his skin. Are there scabs on it? try and pull them off, if there is pus I'd recommend seeing a vet.
