First of all, you don't want to supress an abscess with antiboitics. We got an old horse over pigeion fever very quickly. As soon as it abscesses, fill 50cc plastic syringe with ichthammol, and squirt it in the hole. Ichthammol has amazing drawing & antibacterial power. You will be amazed at how fresh and clean the abscess is in as little as 1 day. I would do this 2x a day. It will heal very quickly.
Often times we will slightly melt ichthammol (no more than 30 sec) in microwave, then draw it in syringe. I cut the end of the syringe so it draws, and dispenses easier.
After a couple days, once we are confident infection is draw out & inner tissue is granulating, we use Resolve Wound to dry it up, and pull it together so it is nice and smooth.
Our old stud got pigeon fever, and this his how we treated it. Then, six months later, he got severely kicked in the shoulder by a mare, and it abscessed. Again, we used the same protocol. Never used antibiotics, and he did not have a scar, not to mention he was over it very quickly.
I can't say enough good things about how powerful, and effective ichthammol is.
The attached post goes into a little more detail, and you may find it helpful. Slow Healing Wounds | ResolveWound