Bad Example for Her Son
Yes, definitely, his fiancee's priority should be her son...but what kind of a mother, what kind of an example is she setting if she allows her son to make her decisions for her?? It's a matter of life and fact that sometimes, the people you love, your family isn't going to. That doesn't mean that you shouldn't love them still and work to integrate them into the family. It can take time, but it can happen.
It is completely understandable that her son is fighting this relationship, fighting having another man in the house. HE was the man in his mother's life, and especially after going through a divorce, he's going to be even more afraid of losing his mother to a new man.
She Should Reassure Her Son
There are a few things she could and should do...one, let her son know that she loves him (and I don't just mean telling him)...let him know that he is important to her, and always will be. It's normal to get absorbed in a new relationship - everyone does it. But she needs to remember her responsibility to her son as well...spending time with just him, and also letting him know the difference between her relationship with her son and with her relationship with you.
Single-Parent / Child Bonding
When a single parent lives with a child of the opposite sex (and I am speaking from experience in the child's eyes), they can form a bond between them that makes them each other's significant other. He has stepped up to the plate, and become the man in her life. He's been the one there every day, and he's had her undivided attention. Now all of a sudden, you are...you have taken his place and he isn't needed anymore. He is going to be scared, unsure of what his role is and where he fits in anymore. He is going to resent you.
She Must Re-Establish A Safer Relationship
Your fiancee needs to establish a new, safe relationship with her son...as her son, and not her significant other. She needs to set the boundaries of her relationship with you, and the boundaries of her relationship with her son...and always, always, making him feel safe and loved.
Be Patient While Things Change
As for what you can do? Be patient...try and understand from his point of view. Remember what he's feeling...he's jealous, he's uncertain, he's afraid of losing the woman in his life. His relationship with her IS changing...change can be difficult.
There isn't much that you can do to make him feel safe...but you can treat him with respect. You can support her in her efforts to make him feel safe, honor their relationship and not try to become a part of it...just yet. Give him time to reconnect to his mom on a new level, and once he's feeling safe with that...he will be able to connect to you.