If they say it's a breed thing, or a small dog thing, be persistent and get them to do an x ray. Insist on it to make sure his lungs are ok. We had a mini with a constant cough ( intermittent) in the beginning. Went on for years and the above was what I was told. She ended up with a calcified mass in her lung which vets in SA thought was unabsorbed tissue from when in the womb. she had part of her lung removed and was on medication for the rest of her life. This is not meant to scare you, just as a warning. Good luck.