Formula help!

My son had been spitting up pretty bad at first so his doctor switched him to the soy similac formula. Well that didn’t work so his father and I switched him to the “for spit up” similac formula (green can) and that did the trick. Well we use wic and wic won’t pay for the spit up formula and his reflux has gotten 100 times better so I figured I’d just use the soy kind again since that’s what his wic pays for. But now he won’t eat more then an ounce at a time (he was eating about 3-4 oz at a feeding) I’m starting to worry he’s not eating enough. Is it just an adjusting period he has to go through to get the old formula out and get use to the new one or what? It’s like he doesn’t like the taste almost. Anyone gone through this and it get better? Help!