You brought something into his home and let him decide where this thing fell in the pack.
When I met my wife, she had a dog aggressive Staffy, that she never let of the lead because it would run away. It's all down the where the dog sees itself in the pack - it was aggressive because it thought held the dominant role above her (which it did) and it ran off because it was the leader, not the follower.
I had him of the lead on our second walk, and never through his whole life had an issue. The dog aggressiveness took much longer, but by the time we lost him last month he was a perfectly calm dog, living with a second dog and was fine with all dogs.
Just make sure both dogs understand you are the dominant pack leader. You don't need to be aggressive and cruel to do this - is recommend simply introducing then on a walk. Giving the dogs something else to concentrate on (the journey), using up some of their energy, and ensuring you maintain the dominant role by keeping them both on the lead for a while works well (even if you normally let them run free). Have just one of you walk holding both of their leads on different sides. If they try to fight (or even interact) at the beginning just keep them moving forward, don't let them interact - get them focused on the walk, which gives you the dominant role. Once they aren't paying each other attention, swop hands so that they are both waking side by side. Any problems swap back, keep walking and try again later. Once they are comfortably and calmly walking side by side, you are the major part firm the road of establishing a nice balanced pack with them.
It's trickier with a brand new puppy that isn't walking outside yet, just make sure the older dog sees you calmly correct him if he does the wrong thing, and sees you correct the puppy if the puppy "does the wrong thing" and makes him uncomfortable. The puppy will be far too young to understand the correction (just a firm but calm NO and move it away - back to the crate of it becomes necessary), but with dogs if the older dog knows and trusts that you as the pack leader will intercede, he won't feel it as necessary to do so himself.
Good luck! I genuinely believe there is no such thing as an "incurable dog", just vastly varying levels of efforts required.