My Son Is 18, New College Student. I Picked Him

My son is 18, new college student. I picked him up from college with a full body rash, fever, and very bad soar throat. The white puss is massive on his tonsils and back of throat. We visited Carle Convenient Care Friday afternoon and his results of strep A and C are negative. It's been 3 days and his fever is being managed by Ibuprofen, throat with gargling Ibuprofen and chloraseptic, and his rash seems a little less inflamed......but I'm at a loss. We are trying to have him seen again tomorrow morning. Any ideas? Mono?