New dog won't go #2...

Hello, all. I am "mom" to a 6-month-old Pembroke Welsh Corgi and a newly-adopted 5-year-old Rough Collie. We picked Winston up from his foster home (where he had been for several months) yesterday, and have been enjoying his company ever since. He is a big boy at 70 lbs, but sweet as can be, very gentle with our Corgi pup, and so well-behaved indoors and out. Everything is going surprisingly well, except for one thing...

Winston eats 2.5 cups of dry food per day, divided between 2 meals. He has been eating all his food, as well as treats, since we brought him home yesterday afternoon. He does seem a bit anxious and overly alert when we take him outside, but will pee after a minute or two. However, after 3 meals and a little over 1 full day, he has yet to poop. We give him lots of opportunities, and he's been on several walks already, but... nothing.

Should I be concerned? I thought it might just be anxiety over his new environment (he went from a rural area to our suburban neighborhood), but I don't want to ignore a potential problem.

He's very healthy, in good weight, and just saw the vet and checked out fine last week before the rescue finalized our adoption. We're taking him to meet our vet and get a checkup on Friday, but I'm wondering if I should call before then.