Suggest Treatment For Ear Infection And Diaper Rash

My 2 year old grandson was put on amoxicillin for double ear infection and developed a bad diaper rash that won t go away. We have been using desitin, probiotics and no milk but it is still hanging around. What else can we do? We were told to stop Amoxcillon 2 days in due to the rash and discomfort my grandson was having and given one dose of Azythromycin but he wouldn t take that either kept throwing it up. We have not given any more medicine to him since day 3 and we are now on day 6 and rash is worse. All juices have been diluted if he has them. What else can we do?