Hi Sharon, it's me Lynne the one with Shiloh. Everytime Jack growls or shows any signs of aggression I would say "Ouch!" and ignore him and act sad and disappointed. Dogs behavior is based off attention, and even negative attention is still attention. This will give him a command that means no aggression or biting, and will take away the reward (attention - even if it's negative attention from you). Also, communicate with him as you would any person or child, and let him know what is going on and that people will be coming and going from the house and that they will come, but they will leave, but that you, aiden the baby, jack, …will stay at the house- make sure you say it in those terms. Also, sit alone with Jack and do a visualization of a house with different rooms I would recommend talking your way through the visualization - "We live in this house Jack and you and I come in the front door and there's a bedroom upstairs, there's a bedroom down the hall, and then as we turn left there's a bedroom that is just for you. Now we're walking in the bedroom and...." Now fill the room up with all his favorite things and things that are just for Jack and that house represents your heart and the bedrooms are all parts of your heart and the baby has a bedroom, and your other child (if you have one) has a bedroom, and Jack has a bedroom too. You do this for all of your animals so that they know that they still have their special place in your heart that is just for them. You visualize it and you verbalize it - animals do understand what we say, but we have to say things in the positive sense if you say the friends and family are not moving in the animals get a picture of friends and family moving in you need to say everything in a positive way "the friends and family are coming (they get the picture of the people coming) and then the friends and family will leave, but me, and baby aiden, and dad, you, and ....we are all staying here together." Animals also communicate telepathically with you, and this is where the visualization helps as well. Try to create a special place for Jack that is Jack's place - a bedroom, a laundry room, whatever it is that has a great bed, and great toys, etc...Let Jack know that you want him to be nice to everybody so that he can get lots of love and be with the group, and if he does not, then he will have to wear the muzzle for a little while. Give him the choice every time, and remember to say Ouch before you would put the muzzle on. When he's being good thank him for being so nice and good and that you love that you can all be together and get along. He's trying to keep his place in the family - he's not being mean he's being scared that he will have to leave because there will be no room for him - he's trying to keep his place and keep your love. Don't be scared of him because then you have just relinquished your alpha position - you are the one in control even when he's doing that. You could also use a timeout instead of a muzzle - and just let him know that is the consequence. You don't want to punish him for being scared, but you want to make sure he learns that it's not acceptable and that he has his special place with you always and that he is loved and will stay with you. Be consistent, and I think he'll be fine. Also, I would get a kong toy and fill it up with peanut butter or whatever he really likes, and that will keep him occupied and busy and having fun, and you could maybe set up a crate with a bed in it and put the kong toy inside it - leave the crate open, but let him go in and be in his little special cave with his cool toy.
~Happy Thanksgiving~
I can't keep a level head when the topic of the dog whisperer comes up, so I just have decided to stay out of it. I will be happy to offer up positive, reward based solutions whenever I can though 
