You need to give some more info on why his stomach is not good
Any time I have had a tummy or gland issue the vet has said chicken and rice
I add rolled oats or quick oats to my mince as roughage and vitamins as standard
Gut health is important you need to be careful after anti biotics and worm regularly
I would look up indepandent reviews on what you are feeding the dog also
and if you decide to change do so gradually
Hope that helps?