Yes, a change in "canned" food could cause this.
Different flavors.......Different ingredients.
Did you just open a new bag of kibble? Sometimes they can be bad, so take that into consideration too.
Did you give him new treats?
I would fast him for 12 to 24 hours, water or bone broth only, then, I would do as Sabis Mom said by using Boiled Boneless Skinless Chicken or Turkey breast (rinsed well) then shredded, mixed with Long Cooking White Rice.
Prepare the White Rice (not instant) as follows: Cook the rice with about 2 times the amount of water suggested. The cook time will take almost twice as long but will make the rice super digestible. Adjust stove heat towards the end of cooking time so as not to burn.
Ratio mix should be 2 cups rice to 1 cup chicken.
Feeding should be divided into about 4 smaller feedings per day. You could even add 1 Tablespoon of PLAIN Canned Pumpkin per meal.
Adjust amount of this mix fed per day for your size dog.
After a few days of normal stool, transition with small amounts of your kibble mixed with Chicken/Rice mix, taking a few days to change eventually transitioning out the Chicken/Rice mix. If stool gets loose during the transition, go back to previous amount fed (where stool was solid) and hold at that amount for a day or so until his gut gets use to it. Then increase again. This is called "bowel tolerance".
Instead of adding canned food, just add some really warm tap water to his kibble and let it set for a few minutes to get the aroma effect going for him which entices them to eat.
You may have a dog with a sensitive gut, which is VERY common in GSD's, so be careful with what you introduce to his diet.
If diarrhea doesn't clear up, have vet do an ELISA test for Giardia as Sabis Mom suggested.