gerber soother

Three month old has been on gerber soothe for two months now and constantly poops gray chalk like color and screams of pain when trying to poop he holds his breath and kinda struggles and I feel so bad for him . Is this normal ? It's been going on for months now so I've been giving him members mark infant the same club brand formula and he doesn't seem to be in any pain when pooping and he goes almost instantly when I give him one or two bottles of this formula throughout the day trying to alternate the formulas so he's not in so much pain also he seems to be having really bad eczema idk if it has anything to do with the gerber soothe but just throwing it out invade anyone else is having this issue . Any advice ? ( I get the gerber soothe on wic so I have to buy the members mark out of pocket )