My boyfriend wants to breed the puppy I just bought him!?

Ok so I just bought my bf a Brittany Spaniel with all his papers and registration and stuff and he said yesterday he was going to eventually get another one and breed them to sell the puppies! I know a little about what y'all have said about BYB but could you please tell me what to tell him so I can explain WHY this is a horrible idea?? He has no idea why its so bad so please don't be rude just tell me what to tell him so I can convince him to have him neutered! Thanks!