Can I change my baby's formula milk? Can Gaviscon cause abdominal pain in a child?

Hello. My son is 6 weeks old. From the beginning I fed him Aptamil milk because I could not breastfeed. After two weeks, he began to vomit (splashing vomiting). GP prescribed us Gaviscon which we have been using for two days. On the first day, however, he did not vomit several times today and there was also crying and zero sleep. Once he falls asleep, every now and then he cries through sleep and fiddies. Are there possible stomach pains after Gaviscon? Could I try a different type of milk. I don't know what to do anymore. It's hard to get any advice from the GP.