Saw A Doctor Today For My 16 Year Old Son

Saw a doctor today for my 16 year old son who has had a temp all weekend, sweated and was dripping with sweat during the night. Extremely sore throat, after a long chesty cough. She did a throat swab to check for strep throat and prescribed erythromycin for ten days. We have to check in a week to see if it's strep throat. But my son is complaining of stomach ache and I feel so bad insisting he has to keep taking them. He is allergic to penicillin. I do give him food before taking them and have only given him paracetamol cos ibuprofen can cause a bad tummy too. Should I persevere or ask for something else?
Thanks, from Nicola.