[matese]

You know your dog best and his reactions to other dogs. Make sure Moose has been neutered, I assume Deuce is neutered, an un-neutered male can be very dominate and aggressive with another male
My suggestion is
introduce them outside of the house, on leash most definitely.
Take a short walk with them so Deuce can get a little bit acquainted with this big strange dog. While walking see how Deuce is reacting to Moose, if / when Deuce is or has calmed down them bring them both in the house, remember, that is Deuce's house.
Once they are in the house I would take the leash off Deuce but keep leash on Moose being held by the BF, let Deuce approach Moose to sniff him out.
I would not hold Deuce, I feel this may cause Deuce to think you are trying to protect him against this big dog, also you yourself are going to be a bit nervous and Deuce will sense this in you.
Do not feed, give treats or bits of food to Deuce while Moose is in the house, sounds like Deuce is food motivated and my get aggressive towards Moose over a little tidbit.
Because Moose gets along with DD little girl does not mean Moose will get along with Deuce as he is a male. If Moose is to frisky and Deuce is not use to being around big dogs let alone big frisky puppies I would watch Deuce very carefully, he could become scared, frightened and go for Moose.
BTW Deuce is a beautiful lil boy.