The bite of fire ants injects formalin and certain other chemicals which are locally damaging to the tissue. This gives rise to pain and also may give rise to an allergic reaction. The shot that the doctor gave, would probably take care of the allergic component. But the pain is likely to have persisted and is responsible for his inability to walk properly. Give him some anti allergic like levocetirizine for a few days and also apply some emollient over the area. It should become ok in a week or so.