I would save your money. What you have going on is likely a yeasty issue and is due to the age of your pup and vaccine protocol. Vaccines suppress the immune system allowing yeast to take over. Your pups immune system has not fully developed (about a yr. old it should) and abx./antihistamines aren't going to help this along. The abx. may help w/secondary bacterial infection...however, a simple trick you can use topically is a mix of Raw organic apple cidar vinegar and water (50/50) and spritz on the area, allow to dry and apply Raw cold pressed organic coconut oil. This will kill yeast and bacteria.
You can also add these to diet. a tblsp. of ACV to water and 1tsp./10lbs of body weight for C.O.
Veteracyn will work on rash too.
Diet change (RAW), grain/carb/starch free
Your vet should know this is a vaccine reaction...shame on them for trying to extract $$$ for unnecessary tests.
Also...Don't give Rabies at same time as other vaccines. And other vaccines, just Parvo and Distemper...titer at one yr old...if there are antibodies, no need to vax. again other than rabies (by - law...sigh)
He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion. - Unknown