What is wrong with my child he is two and a half with advanced language skills. He started stuttering severely and had joint pain and cold symptoms left untreated for 5 weeks. Then got amoxicillen for 10 days and quit stuttering and was fine for 2 weeks.

Stress response. An infection or illness can put a strain/stress on our body. This extra stress can cause young children to have a temporary change in behavior, or even a temporary regression in their development. This often goes back to normal once the stressor has been eliminated. I am happy that your son is back to himself!
He . He needs a complete evaluation and a neurologic evaluation. There are several conditions that can cause this (for example, pandas syndrome and lyme disease, but there are more). He needs to be evaluated immediately. Please write down all of the symptoms, past history, and any medications he has been on and tell your doctor. It is very important that he have a complete evaluation for this.