My son had painful sore throat so i took him to a doctor and he tested negative for covid and negative for strep. two days later he has mild fever and runny nose and cough . is this the same virus or has he caught covid or sth else ?

It is impossible to tell over the internet what the problem might be. Your local physician is your best bet to determine why your son is sick and what is needed to resolve his illness.

I would recommend retesting for COVID 19 since the initial antigen test may be negative. The Strep rapid test with symptoms is usually quite accurate and a culture may be sent at the same time. Treat symptoms of his infection, monitor temperature and breathing. His age was not stated, and if eligible consider a COVID 19 vaccination. Keep him well hydrated, offer soothing fluids.