How do I get my 4 month old regular again?

Ever since I've started supplementing, it's as if my little one can't poop. I try and get his dad to give him as little formula as possible when I'm not with him but he doesn't listen. (The dr recommended it to help him gain weight) he's gone 10 days without pooping. The ped finally told me to give him a glysterin suppository. He went that day, and then not again until 5 days later. Now here it is 5 more days later and to me he seems uncomfortable. HELP!