Muzzle Rash

My Boston has never had any allergies in the past but now has a horrible rash on his muzzle and bald spots on his back. He has a steroid shot. An allergy shot. An antibiotic. He has been tested for mites parasites bacterial infection all negative. Nothing seems to be helping. 

Hello - it sounds to me as though you need to speak to your vet for an appreciation of what treatment is being given and why, and what you can expect from it.  There are very few skin diseases looking like this that an antibiotic shot or a steroid shot would 'cure' - is that what a) you were expecting and b) what your vet was expecting to happen when they gave the shot?   Good questions from your vet might be, 1)  What possibile diagnoses do you think explain these symptoms?   2) which ones have you now ruled out?  3) what could it still be?   4) how do we tell the difference between these possibilities?  If your vet doesmt know, it could be that their lab pathologist or a dematology expert could guide them.  It could be for example, that the remaining possibilities need eg a biopsy to diagose them,or that another scrape is needed before mites or fungal disease can be ruled out.

The point is, that there a lot of things that it 'could' be - autoimmune disease, demodex mites (which need a special deep scrape to rule them out and would tend to get worse after a systemic steroid) and even skin cancer might be amongst the possibilities (bearing in mind that I know next to nothing about your dog).  Guessing treatments and giving them may be an unhelpful approach  - it may be that more detective work is needed.   That is, your vet may want to do more tests, to work out which of many possibilities are causing this and therefore which of many possible treatments are needed.