You may want to look for any cuts in the mouth if he was just chewing it but if he swallowed shards of plastic you may want to see your vet.
Sharp bits could cause damage if he vomits or if his system tries to pass them through him.
An x-ray might tell you where any bits are and how big they are. It is possible if they are small that giving him some fiber will help him pass them safely but is also possible he may need surgery to remove them.
If you can see your vet tonight I would try that first.
If you can't see a vet then I'd suggest you watch for vomiting, blood inthe vomit, blood in the stool or dark and tar colored stool indicating intestinal bleeding, or abdominal pain, refusal to eat, or constipation or diarrhea.
If your dog is vomiting, having bloody stool, appears dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has gums or tongue that look white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, or is becoming lethargic and non responsive to you then you may want to get the dog in to see an emergency vet if one is available in your area.
This site may help you locate one if you need one
Hope this helps you!