How to prevent constipation in a 3-year-old child who can pass stool only after taking medications?

Hello doctor,

My grandson is 3-year-old. He had a constipation problem since he was six months old. He used to pass stool irregularly and sometimes it is after four days or even seven days. When he feels that stool is coming, he sits to pass it out but he will not get success even after trying to push the stool. He can pass stool only after using the Dulcolax suppository and he feels pain during this process. Recently his doctor has advised to give him Muout Plus Powder (polyethylene glycol 3350 USP), one teaspoon OD. Please guide us on whether it is fine. What are the precautions we should take and how many days it should be continued to get the full relief? Can this powder be given by mixing it in dal or vegetables? Does it have any side effects? If yes, then what should be the precautions taken?