I would try a limited ingredient kibble. There are lots of brands out there that carry limited ingredient foods, but a couple of higher quality ones are Pure Vita, Nutrisca, Back to Basics, Nature's Variety, California Naturals.
If you don't want something quite that holistic, there are other "middle of the road" brands that also carry limited ingredient kibbles. Those would be Blue Buffalo Basics, Natural Balance LID, Wellness Simple.
If your pup is having diarrhea on the higher quality kibble, it might be because it's too rich for him. Each dog is different... So if he does well on Hills, then it's OK to stick with that kind. A lot of people feed Hills. It's nothing to be ashamed of. So if he's doing well on it, I say stick with it until he gets older. Then you can try transitioning him over to a higher quality kibble.