The packaging on the bag and can will tell you approximately how much the manufacturer recommends your pup should eat. The website does as well, for example: http://www.nutroproducts.com/nccan-lbpup.shtml
I assume your pup is somewhere around 25-30 lbs? According the Nutro website (assuming you're feeding lamb and rice), he should be eating 2.5 to 3.3 cups of food PER DAY of kibble. Adding canned food to that would decrease the amount of kibble you feed.
Feeding canned alone, you'd be feeding 3.3 to 4.3 cans of food per day.
When I look at what the manufacturer recommends, I have to take into account how much my dog NEEDS to eat, which often isn't as much as we think. An active puppy in the normal household will likely eat somewhere in the middle of the recommended amount. The high end is reserved normally for EXTREMELY active puppies.
If you recently added in canned food, that could be adding to his gastric distress as well.
I assume you're feeding him more than once a day? Smaller meals are easier to tolerate (although pups poop more often the more often they eat)
So, let's assume that your pup should eat the middle of the guidelines. That puts him at about 2.75-3 cups of food a day. That's total food per day.
If I were you, since the canned food might be part of the issue, but he likes it (and he's become somewhat finicky), what I would do is take about 1 smallish spoonful of food (NOT a heaping spoonful). Mix in some lukewarm water to make is soupy, then stir in the kibble so that it's coated with the canned food. So basically, he's going to be just eating kibble, but we'll let him have just a bit of his delicious canned for extra flavor.
If you're feeding twice a day, feed just under 1.5 cup morning and the same in the evening. If you're feeding 3x/day, feed 1 cup in the morning, 3/4 cup midday and 1 cup in the evening. Do that for about two days. Do not feed more treats than usual. If your pup is starving, you'll see it, and you can increase the amount of kibble a bit, up to about 3.0 cups. That should give you a good baseline amount of kibble. You can watch for a couple days then adjust again, as necessary.
Then, depending on whether you want to continue feeding canned as actual diet, or just as a topper, you can either discontinue it, keep adding water and leave it as a flavoring, or add in a tiny bit more when you notice your pup is hitting a growth spurt.
As your pup grows, you'll find yourself messing around with how much he's eating now and then. As weather changes, food intake is affected as well. As exercise demands change (for example if he takes up herding) food intake might change.
My preference is to start conservatively. It's easier to drop a bit more food in her bowl at the end of a meal when my puppy is still obviously hungry than to deal with a puppy that has runny poop for days because I overfed.
You'll get there. I promise!