This is a good question to discuss with your son's pediatrician
(if he has one). The first important thing is to make sure that the ringworm infection itself is totally cleared up, as treatment requires taking medication for a prolonged period of time. Therefore, the first thing I would do is check back in with your son's pediatrician to make sure they agree that the infection is all gone. If not, they can give you some advice about how much longer he might need to take the anti fungal medication.
Assuming the fungus is entirely gone, then it can, unfortunately, take quite a while for the hair to grow back completely, often many months. The important thing is to allow the hair to grow back at its own pace, and to avoid brushing, combing, or otherwise irritating the area, as this might damage the newly forming hairs. If there is a significant amount of itching or dry skin, then applying a simple nonmedicated moisturizing ingredient, such as vaseline or a simple lotion, should be sufficient.
Rest assured that, in most cases, hair recovery after a case of ringworm is complete, even if it does take a while. Good luck, and please talk to your son's doctor