How can I convince my fiancé to take in my son?

I met my fiancé five years after losing the father of my child. Few months of dating, I moved in with him and left my son under the care of my mother.

However, ever since I left home my son has changed. His performance at school declined and he appears emotionally disturbed.  

I feel I betrayed him because since he does not have a father, I should be there for him. I want to start leaving with him but I am afraid my fiancé will also bring his other children to live with us.

How can I convince him? Help me

Hilda