This medication can be used in dogs as a treatment for constipation. However, this is a pretty high dose. Now, the biggest risk in this case with the diarrhea that is very possible is some dehydration or electrolyte imbalances. So, you can use a 50/50 solution to maintain hydration as this medication leaves the system. If you notice lethargy, weakness, collapse etc, I would definitely seek an ER vet. However, this product tends tobe pretty safe and other than loose stool, I do not expect an issue. Here is a website with more info.
I hope this helps and gives you direction.
was it okay to give him the peroxide to induce vomiting though?
This should not cause an issue. I would continue to support hydration if he continues with any vomiting. I hope this helps.