If this can be injected under the skin (as opposed to in vein or muscle) it is quite easy, (I have done this with diabetic dog.) You shouldn't need an epipen, bur could get vet to prescribe vial of appropriate antihistamine from what I read dosage is 1mg per pound, so this should be doable for small dog. If your vet can't help you, look elsewhere. Perhaps a nearby VCA or other animal hospital. You should know location of 24 hour emergency vet nearby in case of off hour emergency anyway, An emergency is not the time to be trying to find one.
Good kuck.
#PetHealthandNutrition