A.M.
Those are two different formula types. Gentlease has the milk protein broken down so it is easier to digest. Prosobee is soy so it has no milk protein in it. He may be having trouble with the milk protein and being on soy can help him out. Did you keep the reciept for your formula? Some stores will let you return/swap them as long as they are not open. I would just stick with the soy and see if you can return the Gentlease. Good luck.