Hi Ally-
Lets back up here for a second before we start assuming an 18 week old puppy has food allergies or a food intolerance. I see a very clear reason for why he is likely having stool issues. You brought him home at 8 weeks old and changed his food instantly and then have subsequently changed his food several more times since then.
It is ALWAYS recommended when getting a puppy to keep them on the same food the breeder had them on for 3-4 weeks. This gives your puppy time to adjust to his new home and environment without the stress of also having his food changed. Find out what they were feeding him at the kennel (NO it does not matter what food it is or if you approve of it or not) and put him back on that. Then slowly after 3-4 weeks or normal stool switch him to an appropriate large breed puppy food. Regular Purina Pro Plan large breed puppy would be my suggestion.
