19 month old has a fever for 4 weeks?

My 19 month old son had strep throat with 103.4 temp 4 weeks ago. He received a shot and a week of Ceflin but was still running a low grade temp daily during daytime and normal temp when asleep. He also had started having a wet cough. He tested negative for strep but bloodwork showed he had a virus. A week later he still has a low temp and still congested and coughing. Dr. confirmed he had a sinus infection and prescribed Augmentin. 6 days later and he is still having the same symptoms. He just broke a canine tooth on the top. What could this fever be? The dr. said we need to bring him back in if he is still running fever in the next couple of days.