If the lab result comes back positive for giardia he will need meds for longer than 5 days. (I assume the vet only gave you 5 days because you did not have a definitive answer yet.) The probiotics will help get his gut healthy again.
ETA - I never sprayed bitter apple at a puppy, only applied it to objects I did not want chewed. Is spraying it at a puppy something you should do? What if it accidentally gets in his eyes?