Is it an abcess or a hematoma? First, get thee to a vet. A hematoma could mean he is still bleeding bc a artery or major blood vessel of some sort got nicked.
If it is a infected abcess(Which takes time to form, not immediate) then he may need it drained and will need antibiotics.
Vet vet vet, and don't feel too bad, injecting the buggers IS hard. I had to give one of mine injections too, from the UNiversity vets, good thing they are so good natured.
Wolfy