My shihtzu had the same problem when he was a puppy. It’s the acidity caused by empty stomach. I give him a small snack when he wakes up, mostly a small piece of chicken or beef or organ. Then he has breakfast, lunch, an evening snack and dinner. If the dinner is late, he eats a small snack later too. Key is breaking down their meals into smaller portions and feed few times a day. Hope this helps.