newborn is constipated help!

So I had my son on the 20th of this month and shortly after coming home from the hospital my milk supply went away and I freaked out n went out n bought a sensitive formula and he’s been on it for about 3 days now and since then my milk has come back with a vengeance but I haven’t pumped it because he doesn’t seem to want it straight from the boob. So I noticed last nite he was fussy and gassy and hasent gone poop since yesturday and even that was hard for him to get out. So my question is what am I able to give him that will help?!? His sleep is being interrupted as well and won’t sleep unless it’s on my chest.