My son is almost 4 yrs old and has been on duphalac lactose to help constipation since 3 yrs,is that harming him & is there another permanent solution?

Long term lactulose: should have no significant adverse effects on your son's health, but dietary modification (increasing fiber and water, avoiding dairy/bananas/chocolate/other constipating foods and bowel retraining should be habits. Long term constipation that does not improve with these measures should prompt a thorough evaluation by a pediatric gastroenterologist.

No harm, but...: Long-term use of lactulose is probably fine, but 6 - 12 months of treatment is usually sufficient for childhood constipation. Regular prevention of constipation includes increased water, more fruits and vegetables, and whole grains/other sources of fiber. Regular sitting on the potty is important as well. Once you have this going, try weaning him off of the lactulose over the course of a week.