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 can understand why you'd be so concerned about this one feral cat and applaud your desire to help him. It's possible that the tail is infected which is why it continues to bleed.
I'm not certain if you're able to clean the tail or not but the following would be my suggestions (in addition to which antibiotic would be the best choice).
1. Gently clean the tail with over the counter, dilute Betadine or Iodine (to the color of weak tea). You could even put it in a spray bottle and spritz the tail twice a day if he won't let you touch it.
2. Amoxitabs would be a good choice for him. The dose for a 10 lb cat would be about 100-150 mg twice a day. Putting it in his food is what I'd suggest (as you plan to do) and hope that he doesn't eat everything but the pill. You could crush it which might work better.
3. Another perfect product for this situation would be Vetericyn Spray which is available at many local pet or grain stores or can be purchased online. It's completely non-toxic and therefore won't be harmful should he ingest any of it.
4. I wouldn't use the Genta spray if it has steroids in it (betamethasone for example) since steroids will retard healing.
I hope this helps and that you see improvement soon. Again, my apologies for the delayed reply. Deb