I plan to not tell my fiance about my son.
What is your reason for wanting to keep such a big secret if you are going to be getting married.
He'd react in a bad way and it would be uncomfortable.
You shouldn't hide something so big from your fiance. When they find out they will be hurt and would want a divorce.
It will be fine. He won't divorce me because he loves me.
Keeping a secret like this while being engaged can be pretty unforgivable. There is no reason how you can be engaged to someone and want to be with them for the rest of your life, and not mention that you have a son. Your son should have been mentioned to your partner a long time ago, so the only thing you can do not to make things right is tell them.