My male got into a fight and now he has a big abscess on his back.
What should I do?

All wounds should be rinsed with Hydrogen Peroxide (H2O2) when they are fresh. This will prevent most wounds from closing and getting infected. Neosporin cream is good for healing minor wounds quickly. Normally an abscess will pop and drain on its own. If not, they normally kind of hard nodules under the skin. If this is the case, wait until a small scab forms and then squeeze gently. The scab will pop open and you can continue to squeeze out old semi-dried pus. Squeeze until all of this is out and watch to make sure more pus doesn't form. Neosporin is okay, but not normally needed if oral antibiotics are used. Tylan or Tetracycline in the water is fine. Amoxicillin from the vet is better.
Note: Some people who don't like using Hydrogen Peroxide, so Charles suggested this: Might I recommend as an alternative, then, Tea Tree Oil, which will also sterilize the wound, but to the best of my knowledge is not acid-based. It also has a really nasty smell, so they don't lick it off (I've had a big problem with that when using aloe on little cuts and bruises; the other rats would pounce on the one placed back in the tank and lick every last bit of aloe off) and the other rats stay away (unless its something bad enough that warrants them being put in isolation, in which case this is not a worry!)