Which formula should I use?

My 1 month started off taking Similac Pro-Advance (which we had from the hospital) and it made his gassy and fussy. So we switched over to Similac Sensitive when he was about 2 weeks and now it seems like he's constipated from it. He'll go 2 days without pooping and he'll try pushing but nothing will come out. When he does go, it's alot. I heard gripe water can help so should I switch him back to his old formula plus gripe water?