A mystery mass of some kind. You'll need to take the cat to a vet for a diagnosis and treatment. The sooner you can get to the vet the better. Sometimes masses change quickly and become harder to treat.
Thank you for your responses it looks like some kind of cyst and I was wondering if I could just give him some antibiotics. Looks like a trip to the vets tomorrow!
Antibiotics may or may not work on the mass. That's for your vet to determine and to prescibe the correct one and the correct dose. Hopefully it's something benign and treatable, either with medicine or by surgical removal..