Switched to soy...please help!

I switched my son to soy after regular formula was making him uncomfortable and gassy. He's been on soy for about four days. I've noticed he's struggling to poop now and when he finally does the color is more green and the texture is more solid. It's not hard but probably the consistency of really thick peanut butter. Is this normal? Do all formula fed babies struggle with digestion and gas? How often should my son be pooping? Is the struggle and thickness something to call the doctor about? Are there other options outside of lactose and soy based formula? Thanks in advance...sorry for all the questions.