Because you caught this very quickly, I would induce vomiting. Of course I am not a Vet, nor did I stay at a Holiday Inn last night. But that's what I would do.
Just pour the HP down his throat, no need to dilute it.. It does take a lot more then people realize to make a dog vomit. If you have a syringe, it makes the HP go down a bit easier. Sometimes it will take 10 or 15 minutes, other times they will puke sooner. It took me a lot of HP before I got my dogs to give up their ill-gotten gains when they got into chocolate many years ago.
Once again, only you can make the decision as to what is best for your dog. We can just make suggestions about what we would do.