Baby diapers

So my 3 week old passes gas like no other. Has no problem with it. But he does strain himself when he has to poop. He did not have a poppy diaper yesterday which my pediatrician said that it’s normal and babies can go a couple days without pooping. But it feels like he is trying to. He just can’t. They told me not to do gas drops or anything that could possibly help. He’s already up to his birth weight. We do the leg exercises and everything to try to help. Pediatrician said when babies try to push that’s when they get fussy we need to help him relax. Harder said than done since he wants to poop. For a minute I thought he may be constipated but then he would poop like normal. Any suggestions?