Ive been taking care of a baby squirrel whos mom was killed after a hurricane How do I make him un-constipated?

We found him two days ago, and all we had at first to feed the poor thing was skim milk. He was dehydrated and freezing, so we fed him that for one day, only to find out the next day that milk is the LAST thing you're supposed to give them. We took him to the pet store and bought him some puppy formula and that's what he's been eating since, but he's only pooped once since we've found him on the first day. I'm so worried about him! Please tell me if this is normal or if there's something we can do to help.. also we know about having to stimulate them to go to the bathroom and we have been, and he's peeing just fine.. but I think the baby is constipated :( please help.Ive been taking care of a baby squirrel whos mom was killed after a hurricane How do I make him un-constipated?
I would take him to the vet. A vet will treat a squirrel - but call first to make sure. He probably needs some fiber in his diet. We were told to feed our dog canned pumpkin, which we mix in his food, this helps harden the stool.