@dk86,
Trying to sort out your post here.
1) Your fiancé, even if you break up, will remain in your life because of the son you have together.
2) That means his mother - who you do not get along with - is also in the picture.
3) The fiancé has 3 kids from prior relationships, but initially only told you (and your parents) about the eldest one.
4) His mother is pushing for you to take in the other 3.
5) You are kinda embarrassed by the lies, which you unknowingly repeated.
You do not have to stay with this man. And, even if you do, God knows you do not have to raise all 4 kids, just because their mothers made poor personal choices. However, recognize that if he wants custody of any of these children, if you marry him, you are getting them as a package deal, too. Plus his mother's meddling ways.
You sure you want to be a part of this?
I am thinking you don't.
Keeping them all at arm's-length, but allowing them in your son's life, seems the best way to keep your sanity in all of this.