It's really hard to tell without seeing him. You mentioned a possible fight with another cat. I'm inclined to think this might be the issue before recently painting your room (only because I just did the same and I have 5 cats and have not had any issues).
It sounds like your cat is definitely in pain. He could have bite wounds (which can be very difficult to see underneath all that fur) and it might not even be from another cat...it could be from a fox, raccoon or dog. There can be bruising and the wounds will begin to close up which can cause an abscess (collection of infected fluid/pus) which can cause lethargy, vomiting and high fever if not treated.
Please have him seen, at least to rule out any possible infections. There's something going on but the only way to tell is to have your cat seen by a professional who can actually check him over.