Help!

Hey! I need some help here! So my son he is 3 now but he was on nutramigen from 9 months and then when he hit a year old we slowly tried doing whole milk but we found out he had a dairy allergy. Now he was on put back on nutramigen toddler until he was a year and 8 months but then the stores stopped selling them and we couldn’t find them anywhere and I started trying non name brand but he wasn’t having it so after his last can of nutramigen I feel like he has had continuous problems with popping. Took him to the pediatric doctor he suggested to try other non dairy milks but none of them he will drink so he is having continued issues they put him in miralax but it doesn’t help anymore. It’s gets to the point that I give him the miralax and I end up having to take him to the ER to get an enema. I don’t know what to do anymore I have tried to change his diet to foods that would regulate his poop but nothing helps. His doctor wants him to get a GI doctor but honestly I feel like every doctor turns him away and tells me his is fine (they have residents at his doctors office) so I am honestly scared to go to this doctor and then turn him away.