Doctor is so unhelpful

My LO has issues with pooping. He is breast fed through a bottle at night and has Gerber good start during the day. He isn't pooping on his own unless we add brown sugar to his bottle. We switched him to similac sensitive to which he pooped on his own but screamed uncontrollably for hours. Put him back on Gerber and no screaming but no poop, and the dr. Just keeps saying to keep trying until we find a balance, but we can't afford to keep trying 16 dollar cans of formula that don't work so we toss them aside. I don't know what to do. I just want him to poop and be full. Is that too much to ask for!?