If your Child is suffering from ADHD then

I would like to suggest few yoga postures for him if he can do it regularly
Sirsasana
Sarvangasana
Setu Bandha Sarvangasana also called Bridge pose
Halasana
Once a week oil massage from neck to back spine.
This will helps in blood circulation to the head and exercise to neck nerves.

Other than this yoga, it’s better he can do skipping or just jump or any other physical exercise which gives massage type workout to neck and head.

Avoid wheat, stop wrapping any food with aluminum foils or using Aluminum utensils.
Green or salad or any natural food which has calcium/zinc/magnesium
finger millet is good for him, also its juice.
reduce or salt and sugary type food and drinks specially Coke and Pepsi
let him have Plenty of water avoid dehydration.
Let him have Arrowroot or arrowroot based food
Avoid all preservative added food/canned food/fast-food/tin food etc.

lastly
soak overnight after washing
1 Walnut
3/4 Almonds
1  tablespoon resins
1  tablespoon Flax Seeds
consume in an empty stomach early in the morning and drink its water. or you grind it along with finger millet and drink it like a juice.

with this 6 months course i can guarantee 50% improvement. if its done daily.