Excessive pooping ?!

I never thought much of it until I stumbled upon a vet podcast where the vet talked about how a dog should not poop more than five times a day. We take three daily walks and he poops three times during all those walks and sometimes more and sometimes also when we're outside in our garden. It's somewhat loose and the last poop on our walks is always a bit runny even. The vet on the podcast said that it could be because the dog doesn't take up nutrients from the food so you need to switch. How do I know what to change to? He's six years old so he's not a puppy, and nothing in our lives have changed, so it's not that. He's always been like this.

We have yearly checkups at our vet and he's healthy. We have even done extra checks on his stomach etc to see if there's anything wrong, but everything is fine, no allergies or so either. He's only sensitive to fat so we are very strict with his food. What food do I give him to make him poop less and for it to be of a normal consistency? I know it's really harsh on their intestines to poop so much. Our vet suggested giving wet food, but it hasn't worked. We supplement with B vitamin as well after our vet recommended to do that until his poops are normal.