Pediatric formula help

My toddler is 16 months (14 months adjusted) and is still on enfamil gentlease that’s fortified (along with his solids). He was born at 2 lbs so we have been fortifying breast milk and formula forever. We’re trying to make the switch to a pediatric formula, but he hates a lot of them. We tried pediasure and he hated it. We eventually landed on Kate farms chocolate but it is giving him the worst constipation of his life. To the point where we were giving him prune juice and apple juice everyday and that still didn’t help and we had to give him MiraLAX (which only worked after 4 days of it).

Needless to say…we’re dropping the Kate farms. Any pediatric formulas/ heavy calorie dense ones that your toddler has liked and not had constipation? Looking for any recommendations!

My cousin recommended fortini, but I’ve never even heard of it before this and ir seems like not many people have used it.