I agree with the trying different brands. It could just be something with Wellness that is not agreeing with him.
As for diarrhea, my little guy is soo sensitive to anything disruptive (too much excitement with friends [human or dog] or any car trip) that he'll get some loose stools. The only time his stool is really really solid is after 3 completely uneventful days.
As for food: On a good eating run, he eats 3 out of 4 meals. But if there's excitement, he might skip 2 whole days. A friend was taking care of his for a weekend and I had to talk her out of taking him to the vet for not eating. Sillly Havs. Remarkably, he's stayed the exact same weight over the past yr (he's 2.5 now), so his self regulation is amazing.
The latest worry - a bad rxn after giving him flea meds. I used the same stuff last year and he was okay. But this year (yup flea season has started), he didn't eat for 3 days. His stomach was making all kinds of noise. When we went outside he was focused on eating grass. We went to the dog park after I thought he was feeling better, and for the first time ever, he growled at the other dogs. He went crazy barking at this one french bulldog - acting like a vicious dog. It was unbelievable. Usually, he's the silent dog that plays with everyone. The regulars there had never heard his bark. I've never seen such a personality change. Anyway - it took a week, but he's back to his old self. But I'm not sure what to do for the next round of flea meds.
This got too long - but just wanted to say my little guy is sensitive to food/routine changes like no other dog I've ever seen.