Constipated new born ! SOS

Brit

My little one is almost three weeks old and is formula fed due to breast feeding issues . He has been crying and gassy but pooping once daily , now this morning I was changing his diaper to find a solid /hard poop which I know is a major issue for newborns because it’s a sign of constipation , we have a doctors appointment tomorrow as today is a civic holiday in Canada . Any suggestions on what I can do to help him until then?