Has your child been sick lately? Acute cerebral ataxia is condition which can occur in children, especially that young as your son is several weeks after an illness caused by a virus.
My sister’s son had chickenpox and few weeks after that he was suffering from acute cerebral ataxia. He was having problems with the movement of his legs and arms. When he was trying to reach for something his limbs were moving back and forth without his will. In his case because it was consequence of chickenpox the treatment was to wait. After few days he was just fine and his movements were normal again. When we asked about treatment options we got an answer that it depends on what is the cause of acute cerebral ataxia.
So if your case is similar to this one stop worry and just be patient because it will pass on its own.