Hi Thanks for your query. I appreciate your concern for your son s allergic rash. As compared to Biaxin the risk of developing rash is higher with the use of Amoxicillin. That is the reason if a person develops allergic rash to Penicillins like Amoxicillin there is a general consensus that it should be discontinued and a Macrolide antibiotic like Biaxin to be given to eradicate the infection. So, in view of the above clarification, the allergic rash is caused by Amoxicillin. This rash will typically appear around the 5th day of using Amoxicillin & will typically last around three days, but can range from 1-6 days depending on the severity of the reaction. If rash is severe or causing itching you may use a mild steroid cream like cortisone for the symptomatic relief. Hope this answers your query. A feed back is appreciated.