Thanks for writing in,
WHat you have described sounds like it may be a cyst or even an abscess being that it is draining this liquid. Given his age though, I would want to make sure it isn't something more serious such as a skin tumor. I would suggest that you bring him to your vet soon and have them take a look at this. What they will do is a cytology on this mass where they will take some cells from it and look at it under a microscope to determine what it is and what needs to be done with it. Given that it is open and scabby he will likely need some antibiotics and if it continues to bother him even if it is benign, then I would recommend surgical removal of the bump.
I hope this helps. Let me know if you have any other questions.
Dog Veterinarian
Veterinarian, Dog/Cat Veterinarian