Thank you for the responses! Yes it's around his shoulder area. He doesn't go all the way outside but there's this backroom he likes to go in that bugs easily come in and out of because it's and old house that we rent with lots of holes and cracks back there. There's no other cats but we do have a friend's dog staying with us for the week. Unfortunately I haven't been able to see if he's been scratching at the area because I saw it on my way out to leave for work this morning so I'll keep an eye out when I get home. I hadn't notice him scratching much before the sore appeared though
I am with fight wound or getting caught on something sharp. Bug bites, even bad spider bites, usually swell and have little indentations in the center. Definitely needs a vet.
Mice and rats can get through holes we can't see, as where the wall meets the floor for mice. Other small creatures, voles and shrews ( who are fierce fighters) can get through small holes too. That looks more like he got snagged on a nail, while trying to go through a hole, or under a stair/board. That has it's own set of reasons to see a vet.