Formula question

So I'm hoping someone out there can help offer some advice. My little guy is just shy of three weeks old. He's been exclusively formula fed since birth. We had been using similac advantage, as that's what was given to us in the hospital. We found once we got home that he seemed to be extremely gassy and fussy, increasingly worse after feedings. He also wasnt pooping regularly. The pediatrician said that was normal as their digestive systems are still so new. We ended up switching him to the similac sensitive. At first it seemed to help but now we're back to what we had been dealing with before. He wasn't pooping, again, so we got the green light to give him prune juice mixed in with the formula. Sure enough, it's allowing him to poop, just not as regular as you'd think. We've tried the gas drops they also recommended and it's just not doing the trick. I hate that the pedi seems to lack the same concern as I do but I am a first time mom and lifetime worrier, but I'd like to think I've got pretty good instincts. I don't know if I should switch him to an even more sensitive formula, one with a better break down of the cows milk proteins, thinking perhaps he's sensitive to that. Help? Suggestions? Thanks in advance!