I want to adopt my fiancées son after we are married for a bit. The bio father is deceased and my question is this. Would my son loose the benefits he is entitled to from his bio fathers death?

Yes, since the new parent assumes the obligations for the child.

I want to adopt my fiancees son after we are married for a bit. The bio father is deceased and my question is this. Would my son loose the benefits he is entitled to from his bio fathers death?

Depends on the nature of the benefits and the state in which the adoption would occur. I adopted my daughter, for example, but we still get Social Security disability benefits concerning her bio-father.