You can sterilize a large sewing needle by holding it in the flame of a match or candle. Likely you could also sterilize a needle or blade with rubbing alcohol. You can also use a scapel, or a new Exacto knife blade. If you make the cut at the lower edge of the abscess, gravity will help it to drain better. The pus will likely be very thick, about the consistency of toothpaste. Hydrogen peroxide squirted into the abscess with a syringe without a needle will help loosen it. This is likely to be a bit messy and sometimes when an abscess is opened some of the pus will come out under the pressure that has built up. You may need help to restrain the rabbit or you can wrap its body in an old towel or the like to restrain it. But two people would make it go faster.
At this point I would not be worrying about a scar. A small neat cut may leave a small scar, but his fur will grow back and cover it for the most part. His health is far more important.