I'd be very concerned about an infection spreading from the necrotic tissue at the end of his tail upwards towards his body. Just because he's on an antibiotic does not mean it's the correct antibiotic for that particular infection (unless a vet did a culture of the end of his tail and an antibiotic sensitivity test to determine the correct antibiotic for that specific bacteria), and soaks/Neosporin will do little for an aggressive internal infection. Yes it looks like a shed coming on, which can obviously cause issues, but his tail has already had an infection and tissue die. So it's very hard to tell sometimes what's going on internally without blood work to get a white cell count, and a culture from the actual infected tissue. It's a scary and frustrating problem because on one hand that dead tissue may just drop off and the stub may heal with no issues, and that bump further up may be nothing, but on the other hand the infection that caused the already dead tissue may have spread upward (or the open, dying tissue may have allowed a new bacterial infection in and it's spreading), and that bump may very well be an abscess. The only way to find out it to have a Herp vet aspirate the bump and check for lymphocytes. If it is an abscess it will eventually cause sepsis without getting him a proper diagnosis and proper treatment.