Just from what you posted, did you just recently switch to the chicken forumla, and did the diarrhea start shortly after the switch?
If so, how fast did you switch foods?
Our resident Dog food expert @Lindathedogsmaid recommends the follow 4 foods to be the best for bullies: Fromm, Natures Logic, Nature's Variety & Merrick - in that order.
Salmon oil is fantastic for the (itchy) skin, and canned pumpkin (not the pie filling!) is considered a super food. With the pumpkin, just a spoonful a day to keep happy & healthy digestive & immune system. along with @Gertie's Mom suggestion, you can freeze some of it into icecubes.
Treats I recommend, more than anything- fresh or frozen veggies. Low calorie, and good for them! Otherwise, when looking at the feed store or petsmart, look for treats that are grain free.