Some dogs are lactose intolerant. Yogurt (unsweetened Greek Yogurt) is a good source of pro-biotics. But it still has fat.
Here is what I give my pup sometimes and my adult Boxer almost every day.
Take a couple of table spoons of yogurt, water it down and churn with a buttermilk churner. You should see the fat float up.
Remove the fat and feed the watery portion (couple of cubes of ice optional). It still has the "good bacteria" in it.
If this works, making yogurt at home is a simple inexpensive procedure.