It's not uncommon to see the symptoms that you are seeing with simple stomach upset. The mucus comes with stomach upset at times as well and it may even look gelatinous.
The first thing that I would want to do is see the vet for a fecal exam and some bloodwork to find out 1) if there are parasites (giardia and coccidia included) or 2) a pancreatic or liver issue.
Some dogs have high sensitivities to things in their diet and the changes can come on suddenly.
So, for the next few days, I would switch him over to a diet of only rice and chicken mixture and you can add plain pedialyte to replace the fluids that are lost through the diarrhea.
Then see your vet as soon as you are able.
Let me know if you still have questions.