This isn't normal, in any way - diarrhea going on and on just isn't normal. He should have a fecal test MORE than once tested for giardia (as giardia hides under scope). Is there blood in the stool?
I would switch (immediately, NO transition at all) over to a moderate food like Canidae (unless you're willing to homecook). Do not feed Hills/Science Diet.
But, I'd really find a vet who can determine what is going on with this baby.
