Second time mama need help

Hi mamas ❤️ soooo long story short (my eldest son stayed on hypoallergenic for most of his infancy and did well on it). Now my 5 month old has been on hypoallergenic for months now and we're looking to switch for multiple reasons. I slowly switched him over to Gentlease about a week ago and now he's fussy, constipated, can't pass gas, colicky etc 🤦‍♀️ what should I try now? Obviously there's something about the Gentlease that's not cooperating with his tummy so what now? Try Sensitive again? What about trying regular ol Pro Advance or something like that? My brain isn't working right now and I need all the advices!