Sorry to hear your dog got into the rhododendrons. Because I knew the plants are toxic, but wasn't sure how toxic, I did a little investigating and found this on the Purdue University website (Purdue University has one of the best animal intensive care units around):
From Purdue University (http://vet.purdue.edu/)
I hope this helps!!.
If the ingestion was recent, you can give him 2 tablespoons of hydrogen peroxide to make him vomit. If that doesn't work, repeat it in 10 minutes.
Monitor him for clinical signs of rhododendron toxicity, and if he exhibits any of them, you will have to find an emergency clinic or will have to contact your vet.