I don't want to spend my saturday night with my friend and his kid
What else did you have planned then?
I'm going to have a movie night with my friends instead.
It is understandable that you want to hang with friends instead of a friend and his kid. The friend might feel hurt thought and it could affect your friendship.
It's cool, he's a loser.
Still I think that you should tell him. He should not expect you to want to hang out with him and his kid, but it would be nice to tell him that you had a change of plans instead of leaving him hanging.