Lido, 8 months old, also has a sensitive tummy. It helped when I switched to a kibble that doesn't have corn, wheat, or soy. I give him probiotics every morning, and the only treats he gets are freeze dried Real Meat jerky treats for pets (for training sessions only). Probiotics help with gut issues, which can affect their breath. Both Lido's gut and breath improved when he'd been on his probiotic for a little while.
Here's a link to the probiotic I use. Lido has to have 3 chews every morning, due to his weight: https://www.amazon.com/gp/product/B0...IBM5VEVL&psc=1