Problems with formula

Has any mom’s used Similac Allimentum ? My son was on similac noesure due to him being born premature. But he reached a certain kg and the milk gave him REALLLY bad tummy problems so they agreed to take him off and put him on Gentlease. Well that works a little better but he’s been stopped up for 5 out of the 7 that he’s been on it. I’ve given him gripe water to help him poop and earlier this morning when I gave it to him, he hasn’t pooped yet. He grunts super loud and cries after each feeding and has problems with sleeping bcs of the pain. Idk if it’s a intolerance or an allergy? I read that allimentum is for protein breakdown sensitivity or for fussy babies after feeds. Has anyone put their child on it for this reason also? If you use it, how does it work? Can you give me feedback?