To correct my daughter was on vancomycin not erythromycin.
The only time your child could have reoccurring mrsa is if he is a carrier and they will test it through the nose to see if he is a carrier. And the only way you can get mrsa is when you touch the infection and put it in your mouth. I'm giving you this information because sadly my first ended up with mrsa at 9 months old because my roommate had a bandaid over her soar and left it on the couch and my daughter put it in her mouth. She had it on her butt cheek. She was on erythromycin for two days in the hospital. Also it wasn't in her blood stream which was good.
tezza, I'm sorry for your worries.
I think this is one of those questions you need to ask your doctor, and they would be willing to sit and answer for you.