Hi donnamarie, there are people on here who know far more than I do about dogs, from years of experience...
I do know, however, that dogs develop their own dominance order-so it might be that even though your partner's new pup is younger, it turns out that he is dominant! I am told it is unwise to try and interfere with this. Why are you keen for yours to retain dominance?
Two boy dogs, especially of such a stubborn breed, might be inclined to fight though, and you will have to really watch them together.
Give them both a cuddle from me, I do love SBTs :)