Hi Natalie, my cat was bitten a few years back, the bites were very similar to what you described. I rushed my poor sweetie to the vet trying to imagine what could have bitten her, dog, fox, bear, tiger...honestly it all went through my mind! The vet took one look and confirmed it was a cat bite, so my guess is that it is a cat that has attacked your cat. I am taking another guess that there may be a new cat in the area that is staking out it's territory and has decided to cross paws (and teeth?) with yours? If you have not taken him to the vets, it may be a good idea to, the vet I am sure will have a good idea of what has bitten him and will give him some medical treatment in case it may get infected.
If you do find out that it is a cat that has bitten yours, I would recommed that you continue to let him out once he is fully healed, I sure that they will soon settle their differences! Best of luck, keep us informed on your cat's progress, hope he recovers soon, Sue