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I am sorry your cat is not feeling well. There are many things that could be going on with him. However, it is important to figure out if he is stable and not requiring urgent attention. So it is important that he has nice pink gums, is not breathing heavily (using his tummy muscles) isn't panting, is drinking and is responsive, alert and mobile on all 4 limbs. If any of the previous points are present then he should be seen by a vet as soon as possible. Cats are very good at hiding serious injuries so something mild can be very serious.
If he is alert and happy then it may well be he has gotten himself into a fight over night and has an injury from this or is developing an abscess. These cat bite abscesses are very common and show initially with the cat being under the weather, quieter and off their food, often this is because they have a fever. If you are happy that he is stable and well, you could try a warm poultice on the sensitive area as this may soothe the area. Offer warm smelly foods such as chicken broth, tinned pilchards in sauce etc and then if stable/better get him checked by your vet on Monday. Unfortunately, there are no safe human or over the counter pain relief medicines we can give to cats, so I cannot advise you on this.
Good luck and I hope he recovers soon