What Does Fever While On Amoxicillin For Ear Infection Suggest?

My son has been taking amoxicillin since 3/8 for an ear infection. On 3/13 he started a fever that has continued thru tonight. We took him into urgent care on Saturday night and the dr said he probably has a virus but that the ear looked ok. Do I take him in tomorrow? How many days till a fever becomes a concern?