Small lump

My 6 month old kitten has a small lump on his side. It is about the size of a pea, and it is soft. Should I take him to the vet, or could it be the result of an old fight?

You are a good Mommy to have found the lump.
It is probably nothing serious but I think that it is essential you have it check out by your kitty's vet.
A lump is not something you want to be guessing about....especially in cats.
Better safe than sorry.