Since Calvin has a sensitive stomach, I'd look for an LID (Limited Ingredient Diet) type of kibble with only one protein and one carb source (the ones I recommended are, I think) so you don't trigger a bad reaction. Some of the foods that have been recommended are great foods, but are multi-protein/multi-carb source kibbles and very rich (ex: Orijen and Merrick) and may not sit well with ole' Calvin's tummy.
One of my dogs is very sensitive (skin rather than stomach, but same concept), so I feel your pain Pops! I've also had boxers in the past with sensitive stomachs, and I had to feed them LID diets, too (and not too high in protein/fat either).
