I had a similar situation with my ex boyfriend's dad's pitbull and my yorkie. Whenever they would attempt to play together the pitbull would bark or try to attack.

The way we solved it was fairly simple. Tie the aggressor between two trees with 2 leashes (about 8 feet apart). Allow the smaller dog to approach from behind while on a harness and leash rigged to a pulley. Rub the large dog and sing or play while your S/O rides a bicycle around the trees while shouting to distract both the trees and dogs. Make the smaller dog bark twice and the bigger one bark once.

Me and my ex's boyfriends dad are mareied now and haven't had a fight since. :)