Thanks for the additional information. I really appreciate it.
Since he's having problems defecating, we have to be concerned with the possibility of an obstruction.
This is what happens when the dog eats something too big, too bulky, or too long to make it through the digestive tract and gets hung up somewhere. The fact that your boy does have some of the plastic sticking out of his bum and is both lethargic and is having problems defecating, the worry is high that we're dealing with something like that.
I absolutely, positively have to recommend that you have him seen by a vet tonight. If there's an emergency vet in your area, I would suggest going there. If you're not sure if there's an e-vet in your area, you can go to: http://www.vetlocator.com and plug in your zip code. They'll point you in the direction of the one nearest to you.
If there's no emergency vet in your area, then you can continue to monitor him until morning, and then give your own vet a call and have them see him. He does sound like he's relatively stable, so if you'd like to wait to see your own vet, that's acceptable, but keep in mind that if he takes a turn for the worse, then you'll need to have him seen at the emergency vet.
I hope this helps.