My sister son boy 3year old had lactose intolerance we cut all the milk product what is the treatment of this condition?? is long life problem

See below. Eliminating lactose-containing food is the treatment. If he is better with that dietary change, stick with it for now. Your sister can discuss re-introducing dairy in the future with his pediatrician. Some children do "outgrow" this issue, but some certainly have it through their lives.
Cookies and milk. Lactose intolerance for some is a life time health problem but for others it seems to only last a few months or years. By using alternatives like rice, soy, or almond milk you can enjoy your cookies and milk. After time passes ( say a year or two) try regular products again. Start with yogurt and cheese first. Hopefully the intolerance goes away.