Constipated 3 week old

I believe my son is constipated. He is formula fed and been on the same formula since he was 5 days old. Hes poop very little in two days yesterday it was just a small amount and today it was a small amount but on the more solid side then usual hes also struggling to push. Hes been eating fine since his last feeding when he decided to throw it up about 15 minutes after eating. I sent my husband to get pedia lax glycerin suppositories per his dr. Has anyone had experience with those suppositories or other ways to help him poop?