Is he uncomfortable with you touching it?
I remember totally freaking when I found a pimple on Clevelands tummy as a kitten, how embarrassed was I when I was told by a rather amused vet that it would be his nipple, along with the otehr pimples I thought was a skin out break. For some bizarre reason I didn't think male cats would have any!!!! Obviously a very senior moment
Yes, any changes to the skin are an issue, it could a miriad of things. Maybe you can take a picture? But if it's an irregular shape or he's uncomfortable, but even if he's not uncomfortable, take him in anyway.