Formula Change? HELP!!

So my son Noah was born at 34 weeks, 2 days on 12.24.17. I stopped breastfeeding while we were in the NICU, & they put him on Enfamil Enfacare. The formula was causing him really bad reflux & constipation problems, so I asked them to switch him to the Enfamil ProSobee, a Soy formula. The ProSobee was great on his stomach; no constipation, no reflux. BUT, he wasn’t gaining any weight, so they switched him again to Enfamil Gentlease, which he has no reflux issues but has so much constipation to the point where he’s up all night trying to poo, crying & straining. I would like to change back to the ProSobee, but I don’t know how to help him gain weight with it. Any suggestions???