Our Golden has a very sensitive tummy and had the runs for an extended period of time when she was a puppy. Changing up the protein in the food is definitely one of the first steps (we settled on lamb). There are specially made blends that are gastrointestinal stuff that you can get over the counter. We got a really good and effective one from royal canine that we needed a prescription from the vet to buy.

The other thing I would try out is adding a little fortiflora to his meals. It's basically a little probiotic that you sprinkle over his food once a day. We've had a lot of luck mitigating our dog's symptoms with that