Welcome to the forum! I'm sorry to hear of another family being subjected to the deliberate emotional manipulation that is the Watchtower's shunning doctrine.
Your fiance needs your support. He is the focal point of the pressure. This maltreatment is designed to coerce him back into the Watchtower fold, along with your son and you too if at all possible. It really is their doctrine, it's in their publications, and they really do say it's intended as "correction" for misconduct.
He needs you with him, partners united against the pressure. For the sake of your son, don't allow his mother to dictate your family relations. As others have said, you are your own family now, and it's a package deal: she can accept the entire family, not just parts of it. It's important to start now, the longer you delay the harder it will be to have her respect your boundaries.
Good luck to you and your new family--may you live happy and free!