While vitamin D can be toxic your dog is not in any danger from swallowing 1,000iu, I say this because the maximum safe level of vitamin D is 10000 IU/kg which means that your dog is not in any danger. Problems only occur with this vitamin if it is given repeatedly not as a one of dose.
So in your case you need do nothing
Call the veterinarian - ask them.
No vets on here [ yet, anyway ]
It won't kill him if that is what you are worried about. If anything, it will do him good. That is not to say give them to him all the time-they get all their needs in dog food. Don't worry. Just don't do it again.