My 2 Years 8 Months Old Son Suffering With Constipation.

My 2 years 8 months old son suffering with constipation. Pediatrician prescribed Mu out+ (Polyethylene Glycol 3350 Powder) of 8.5g. After using that he is going to the motion regularly. In between, when he got fever we stopped giving for 5 days. Again constipation happened during those 5 days. Now we are using this powder continuously for more than 2 months. How long does it needs to be used?

Today morning after drinking the solution, he vomited 3 times. Does these vomitings are due to this Mu out+ powder? Can I use Domstal 3.5mg for him?