Unfortunately, a big part of your problem is what you're feeding him. ProPlan is full of fillers, byproducts, and other poor quality ingredients. If you feed a better quality food (I'm not familiar with the one you're switching to but I'm assuming it's better) you generally will feed less which equals less poop. It does sound as if your dog is going a lot more than normal.
I second the digestive enzymes and I would also add some probiotics. They are two different things that you can use together to help his digestive system function more efficiently. But the most important thing is to get him eating a better food.
Applaws is much better, here's a link to the ingredients.
http://www.whichdogfood.co.uk/dog-food-reviews/0186/applaws-puppy-small-and-medium
My vet suckered me into feeding my dogs ProPlan, but I have since done my own research and found its an expensive mediocre food. Lets hope Applaws sorts him out and makes him more regular again as I cannot potty train him when he's all over the place and unpredictable.
