On Boundaries

I am a single mother and recently had my boyfriend move in to live with me and my son.
My boyfriend got upset with my son on a day that I was out at work and slapped him hard, kicked his girlfriend out of his room and cursed him.

When I heard that I came home and told him to pack his things and move out.
He argued that I never set a boundary that he cannot lift his hand on my son if he needs to be straightened out. I said that the way I was raised and the way I am raising my kid, I never thought this would ever be an option, especially when it is not his son.

He is asking a second chance but I fear that his judgment is that it’s ok to use physical punishment and in my mind it was the wrong way to handle it.

How do you handle a cross boundary that was never set?
Like, he could come tomorrow and say, oh, you didn’t say I can never punch you in the face, now I know…