Cant switch

So my son's pediatrician changed his formula from gerber good start to soy because he was spitting up to were he drenched his clothes, has bad gas and was super fussy and said to give it to Monday he was doing fine till Sunday then he started spitting up again so we took him in today and the pediatrician said he wasnt worried about spit up and because hes not losing weight and said he couldn't switch him cause wic only covers Gerber which isnt true he said to stay on the soy which caused me and my boyfriend to argue cause he wants to get a different brand of formula but I told him not to because we dont know if our son will have a reaction to it I dont want to waste money on something he can't eat and we cant afford to buy formula everytime we need it