What antibiotics can you take if your allergic to bactrim and amoxicillin?

My son is allergic to amoxicillin, but he can take azithromycin (Zithromax) with no problem. The nice thing about azithromycin is you take a double dose the first day, then a regular dose for the next 4, so you're taking medicine for only 5 days instead of the usual 7-10.