Newborn size advise

Alaa

Ladies my newborn baby boy now is 6 weeks 4 days and he still fits into newborn outfits its not super tight not to wear them but I keep saying this is the last day you would fit!! Is my baby boy not gaining enough weight to go up in size.. I also wear him 0-3 and looks fine. He was 7 pound 2 ounces when he was born but I'm still waiting to see him get chubby