Give him prune juice (unsweetened one). Dilute it with water (1 part juice and 2 parts water) and give it to him. U must also give him water throughout the day, not only just during meal times.. you can also try probiotics and yogurt.
Must giver water. Its very important. Teach and start from young if it persist go see a doctor.
you can start giving him water through the day. give some prunes purée. It works!
Give some apples or other fruits everyday. Need to give water too.
As and when you have to give water
Prune juice works wonders.
Prunes!