Don't force him to play with the puppy -- that's making it worse. Can you put baby gates up in the bigger house? It might help if Milo can see the new pup but knows the pup can't get to him. Treat him for calmly watching the puppy, and vice versa. They need to get used to each other's presence without any contact. After a little while, you should get a better indication of whether they'll be able to interact in the future. Never leave them alone together when you're not home, regardless -- even in play, a large dog can hurt or kill a smaller one.