Treating EPI (Exocrine Pancreatic Insufficiency)

So I have always had rescue mutts and never had any health problems with any of them. Ever! This time around we got a English mastiff from a breeder and spent the money for what I thought was supposed to be a akc healthy legit puppy. Well the last 2 months have been a 2,000$ headache! He has had explosive diarrhea since the day we got him. We put him on 4health (tractor supply brand) large breed puppy food. That didn't help so we took him back I the vet to find out he has really bad roundworm. Took care of that but still the pudding poops (ruining the brand new grass we just spent thousands of dollars excivating my whole backyard! Which is the least of my worries but still) so we took him back to the vet and she said she suspected EPI (Exocrine Pancreatic Insufficiency). I looked online an he has every simptom of this terrifying disease. My poor puppy is literally starving to death. It is going to be $300 to get the blood work done to 100% diagnose him. Well after sucking up all this money to find out for sure that he has EPI I'm wondering if anyone has any idea of the road ahead of us. Has anyone else had a puppy go through this with any suggestions or more information on how to live with it and treat it and the costs. The vet has us feeding him boiled beef and rice and pumpkin which I'm not sure is even necessary because he still has liquid stools. HELP!