Hi, I'm Dr Steve. I would like to help you with your question. From your description it sounds like he is having pain from something in the back end. If he did have an old cat bite, it is possible that an infection could be causing him to have some discomfort in that area. If this is the case the antibiotics should help. It can take several days to a week before he feels better if it is an infection.
The other possibilty that I see , especially if he is an older cat, is arthritis in the hips that can cause discomfort. If he is an older cat ( over 10 years) then treatgin him for arthritis may be beneficial. One other thing that can cause pain in the hind end and licking is a urinary tract infection ( UTI) . I have a few questions for you and then I should be able to help you a little better. Do you remember what antibiotics he is on now and when he will be done with them? How old is he? Is he having trouble urinating or going to the litterbox more frequently than usual? If you can answer these three questions, I will be back online around 6PM central time to see your replies and then I can give you some more specific information. Thanks for your patience and understanding.