My 4 year old has broke out in a rash trying 3 different kinds of antibiotics. We see an allergist in a month. What are his options if he's allergic?

Discuss w/allergist. It's pretty unusual for a 4-year-old to be allergic to three different classes of antibiotics (i assume that his physician switched to a different class after each rash). It's much more likely that the rash was a manifestation of the infection, and not a reaction to the antibiotic. Your allergist will sort this out.
Too many antibiotics. Overexposure to antibiotics is a common problem caused by parents taking their children to urgent care clinics and ers where they are seen by non pediatricians.