It may be a sebacious or other type of cyst. The best thing to do, if it is growing, is have your vet take a look at it and possibly remove it before its starts to bother him, bleed more or refill up.
Disclaimer: The above is provided for informational purposes only. As I am not a veterinarian -- the above should not be considered, or relied upon as, veterinary medical advice. I strongly encourage you to consult your own personal Licensed Veterinarian regarding any serious animal medical matter as they will be able to see and exam your pet.