lump on my pet rats back

Question
I just noticed it yesterday when I was cleaning his cage. He is about 1 1/2 years old he can be handled but i am afraid to poke around on the bump (he has bit me before) it is about the size of a grape and has a dark colored liquid that ran out of it. He doesn't seem to be in any pain. I can not afford to take him to a vet I was wondering if I could give him some sort of antibiotic if it is an abscess.

Answer
 If it is oozing then it is almost certainly an abscess.  Though antibiotics are the best way to treat it, I wouldn't recommend giving him something yourself unless it was prescribed by a veterinarian.  You may be able to get away with cleaning it with an antibacterial soap made specifically for wounds (most pharmacies carry it).  If you scrub it really well several times a day and use warm compresses to help drain the pus it may go away on its own, depending on how bad it was infected.  If this doesn't help or it gets worse you'll have no choice but to take him to a vet.  It shouldn't cost all that much for him to be seen, the exam should be around $35 and the antibiotics, depending on the brand, can run anywhere from $8 to $25.  I hope he feels better soon, good luck!