Advice on buying a second male dog

We have a 5 year old Flat Coated Retriever who has not been neutered, and won't be. He is a very friendly dog except with a few other males, not all entire males but some, he has had a few fights.

We want to get a second dog, a puppy, male, another big breed, either a similar size or bigger, I know our dog will be fine with a puppy my concern is when the puppy is older if they have issues with each other?

We will be buying from a registered breeder and wondered if they would be able to help to select a male puppy who is less dominant? Is it possible to tell at such a young age?

Any information from people who have two or more entire male dogs would be great......we would never neuter either dog or consider a female so really want to find a way to make it work with two males. Thank you.