5 week old not pooping

Okay so it has been over 12 hours since he has had a bowel movement. And even before then it wasn't much maybe a quaternary size amount each change. He is passing gas and eating well as well as has wet diapers. He's fussy when he is straining and I can tell he's trying it just won't come. What can I do? He's breastfed but was being supplemented with formula but his pediatrician has stopped that now since he's doing good with gaining now. So mama's please help!