Formula Frustration - What should I do? I can't find a formula to suit my little one!?

So my little one is 5 weeks old and is on his 3rd kind of formula. At first they had him on Similac Advance he was on that for 15 days, until he started having projectile vomiting and horrible gas issues, along with constipation. They then put him on Isomil advance and prune juice and he was on that for 14 days, his vomiting improved and the gas improved a little, but the constipation did not. So they then switched him to Enfamil Gentlease Lipil and told me to put karo syrup in his bottles, the spit up is about the same and the gas is pretty horrible again, oh and the bowel movements have not improved either and he has been on that now for over a week. I now have to buy formula again and I'm not sure if I should continue with what he's on or switch to something else? I was told to give Similac Sensitive a try...What do you think? Thanks! (all this switching formulas have been under doctors orders)