Thank you for that information. The first thing I would suspect it is the change in food as some dogs do not tolerate this well at all. It does coincide with the time you started a new food and the time it would upset the stomach to this degree and the fact that he is not eating as much.
Drinking is great to keep him hydrated so let him continue that.
What I would try is fasting him for 24 hours, then give a bland diet of boiled chicken and cooked white or brown rice. The fasting will give the intestines the time it needs to become less inflamed and the bland diet will help with digestion. If you want you can even buy Hills dog food which is usually what your vet would recommend for a bland diet.
Give two or three tablespoons of the diet , wait a few hours for him to digest that and then give him a few more tablespoons. Then start adding in the regular dog food to the diet.
Usually when we switch dog food we like to do it slowly , for some dogs it can take over a weeks time of mixing old with new before they get used to it.
At this point your vet may not want to give shots until the diarrhea stops.
Another thing to consider is parasites and protazoa which can be picked up in parks. Your Vet would need a stool sample to determine this. Each is treated differently.
So I would start with the bland diet over the weekend and see if you notice a difference.
If things seem to worsen , if there is fever, vomiting, or the gums are pale or white then seek Vet help.
Here is an example of the Hills digestive dog food http://www.hillspet.com/en/us/products/pd-canine-id-dry