Vomitting anything he eats. For long?

My child on 4th August 2020 in sleep vomitted out every thing he has eaten & that night he vomitted 8 times. Next day we Consulted a doctor and he asked us to take ondem and nutrolin. We gave him this however after 2 day here also started having loose stools. So dr gave him Metrogyl 3 times 3 ml along with gelusil and ondem Dosage suggested was Ondem before meal gelusil one hour before meal and Metrogyl one hour after meal. Now 3 days after Metrogyl his stools are becoming normal slowly however today when we showed dr he suggested we should give ondem only when he is having vomitting. So we didn't give ondem today and he has vomitted everything he ate for dinner. My question is what should we do now? So we continue ondem, if yes how many days? We will be giving Metrogyl for another 2 days and completing 5 days course of antibiotics. However wanted to check what to check? What food should he be given? Is ondem safe to be given to a child for long? What could be causing this issue