Best diet for dog prone to diarrhoea

Our customer asked:

My cocker spaniel seems to get bad stomach bugs regularly but the vet has been been unable to identify the cause so far. They tested inconclusive for pancreatitis. He's 21 months and has been on Arden Grange since a pup and loves it. He does have some dandruff now as well. We have cut out all non-doggy treats and he mostly gets boiled chicken as his kong treat so it's all healthy. He will get in to mischief and scavenge on walks so we are working on that.
New vet thinks we need a lower fat food but I want to make sure he doesn't lose much weight as he's perfect 12.5kg now.
What do you recommend?

Our reply:

Generally for dogs like this I prefer to try one of the good quality diets which has no cereal in it. Some dogs do much better on diets like this and they contain just as much energy as Arden Grange. I would suggest Barking Heads Fusspot food to try for a few weeks and see how you go. You should pick one or two treats which have just a single ingredient and use those as rewards, but allow no other foods at all if you can manage it for a while. This would give the digestion the best chance to settle down and get under proper control. The Thrive ProRewards Treats are very good. All these products are on the website.