The book "What To Expect, The Toddler Years."
Your Fiance has to remember, that HE is the adult, and the child is a child.
The child, is not fully developed... emotionally nor physically.
The human brain, is NOT even fully developed until 26 years old.
It takes, ALL of childhood, for a "Parent" to teach their child, things.
"Expectations" of a child, HAS TO BE, age appropriate.
Otherwise, your Fiance, will continue to be, too hard on the child or your Fiance will continue to get frustrated, and take things personally.
Being a Parent, means, LOVING your child and accepting them, even when they do not do what "you" want. That is what a parent is... it is NOT situational. It means, being there for your child, NO matter what. Good or bad. It is not just "playing" with a child. It means, being a good role-model and teaching your child things, and realizing, that a child is still learning... from you, good or bad. It is a 24 hour, 7 days a week thing.
KEEP in mind, that at 2 years old, the "emotions" of a child is NOT EVEN FULLY DEVELOPED, yet. And they don't even know how to communicate yet. Not even some adults have that ability. A child ALSO needs to be taught... about the "names" for feelings and how to communicate. They do not know how, yet. A parent teaches them that.
A child, is not a dog, that you train or punish, like a pet.
It is a child. A human being.
Your Fiance, CANNOT expect, a child to be, a robot.
HE has to, learn to parent.
And how to accept, a child for being a child.
Otherwise, he will have a rocky relationship, with his child.
And that is not good.
It will affect, the child in adverse ways.
Your Fiance has to realize... how to be a parent. He is full of expectations... but does not know how... to parent.
Your Fiance ALSO has to realize... that kids make mistakes. Unintentional. NO KID IS PERFECT. That is what childhood is for. To have the parent there for them, and to GUIDE them. Not just expect things.
I really hope, your Fiance does not turn your son into a pent-up frustrated, boy, who can't even say how he feels. Because, your Fiance seems to "quash" your son and a child's natural spirit.
Boys... NEED TO LEARN, how to communicate and how to express themselves. Just like a girl. Otherwise, they will be, rigid pent-up emotionally frustrated, individuals.
The stereotype of boys having to be "strong and silent" is a real detriment.
You said, your Fiance is great with your son, as long as he is behaving. Well, that is really not very nurturing, for your son. It is negative.
Kids... no matter what, has good days and bad days. JUST like an adult. You cannot just "love" a child, only when they are perfect, and reject them when they are not.