How much PPP were you feeding him? The reason I ask is that BY FAR the #1 reason for puppies to have diarrhea isn't intestinal parasites or stomach sensitivity, it's over-feeding. And #2 way down is intestinal parasites, which are notorious for eluding tests and being difficult to eradicate.
A tiny bit of Tylan is so strong, if that doesn't clear him up real quick, I'm sure you've got a problem other than sensitive tummy.
The reason I ask is that 14-week old Golden puppies are usually trash cans, eating anything that will go in their mouths. Their tummies aren't particularly sensitive, like some breeds. But lots of people -- vets included -- immediately assume the problem is a sensitivity issue or protein allergy, when statistically it's usually something else. And, of course, #1 and #2 are over-feeding and parasites. The former is easy to correct, the latter extremely difficult to diagnose and eradicate. But both of those are far, far more common than sensitive tummies. Food allergies are one of the most common misdiagnoses in dogs. Just something to be aware of. I've seen people at wits end with their food allergic or sensitive tummy dogs, only to have the problem clear up in a jiffy when it is properly diagnosed.