I need to find the right formula, my baby's stomach is upset!?

He started on Enfamil Lipil and was extremely gy, but otherwise good. I switched him to the Gentlease and he seemed better at first, but poop was green and very loose....and then he would just scream for no reason. Then tried soy, and he seems to projectile vomit and not poop at all. He also acts like he is chewing on something at times after he eats. He's not throwing up every time but 2-3 times throughout day and always seems hungry, which could be reason for vomiting. I don't want to continue to switch formula, but if you don't try different ones, how will you know what works best? I may be doing more harm than good. My ped wants me to put him back on reg. enfamil, but I don't think that would be good. I am so uneasy and confused and feel bad for my baby. If you have gone though this, please help and share!