Best formula for belly troubls

Right now I’m pumping and formula feeding as well. For two weeks he was getting 1 oz formula and 2 oz of breastmilk in each bottle. We are trying to transition to straight formula eventually. Now we are doing 2 oz formula and 1 oz breastmilk. Two days ago he didn’t poop. Then yesterday he let out a ton finally. Poor kid has belly troubles anyways. He seems to have trouble passing gas and pooping. The poop wasn’t hard or anything. Now today he hasn’t pooped at all and is straining a little and getting so upset because his belly hurts. He’s let ou has just no poop. I feel so bad. Right now we’re on enfamil AR . I’m wondering if enfamil gentlease would help better since this one is thickened a little??? Please help!