My son was diagnosed with tonsillitis and received antibiotics. he finished it on wednesday and sunday started with sore throat. what to do next?

Hard to be sure: It would be valuable to know if a test was done to show he had strep A, which would have been the only reason to give antibiotics as well as which one you used. Did the first round fail or was it viral and now you have a real strep or another viral infection? A strep test and a different antibiotic if it is strep should sort this out.