Hello, I'm Dr. Deb.
I recently came online and see that your question hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
I'm sorry that this stray cat has been injured but hope that he'll let you treat him if at all possible.
There are a number of antibiotics which can be given to cats for an abscess. I'll assume that he weighs about 10 lbs which the weight for the average cat and based my dosages on that weight:
1. Clavamox at a dose of 62.5 mg twice a day
2. Cephalexin at a dose of 100 mg twice a day
3. Amoxicillin at a dose of 100 mg twice a day
4. Cipro at a dose of 100 mg once a day
5. Clindamycin at a dose of 100 mg once a day.
I typically treat abscesses of this sort for a minimum of 7 days or longer.
I would also gently clean the wound with over the counter dilute Betadine or Iodine (to the color of weak tea).
Another good wound product would be Vetericyn Spray which is safe even if he licks it off. This product is available at many pet or grain stores or online.
If you have access to other antibiotics than the ones I've listed, let me know and I can provide the dose.
I hope this helps and that you see improvement soon. Again, my apologies for the delayed reply. Deb