What could be causing weight loss and sores to develop on my cat?

I have a 5 year old neutered cat named Jinx. Several days ago, my mom and I noticed what looks like a bite wound, very close together on his elbow. It sort of reminded me of a vampire bite (I know he wasn't bitten by a bat, he's strictly an indoor cat). I also found a pea sized wound on his side. It almost reminded me of a cigarette burn. My mom and I have 5 other cats, but they don't play so rough that they could cause the wound on his elbow. Yesterday I saw he had just a single (puncture?) wound on his other elbow, and now one on his neck. I have no idea what could be causing these sores. Not only that, my mom has noticed (and I agree) that Jinx had lost a bit of weight. Does anyone know of any diseases or illnesses that could be causing this? His personality isn't any different, and he's not lethargic.