1. Massage scalp with potato skin every day.
2. Boil a cup of potato skin in water for 20 minutes and then strain the liquid. Use this liquid to massage the scalp, after an hour wash hair with mild soap.
3. You could use henna to temporarily hide the grey hair until his grey hair reverses.
4. Make curry leaves paste and apply on scalp, leave it on for a while and wash off.
5. Apply coconut oil every night before bed.
Give him healthy food, less carbohydrates. I think you should talk to a dietitian and get his food chart for best results. I understand how stressful this could be for you and your child. Prayers.