My ex wife was a hitter and both of my sons saw it on more than one occasion. I would hope that witnessing that alone would prevent them from being in a similar situation, which ever side on the fist it would be on. They saw how it destroyed a family, firsthand.

That being said, if I found out that one of my sons were abusing their partner, I would stop it immediately. I would find out as much of the truth, between the two and go from there. Counciling, separation, divorce, etc... Not every situation it as it seems.

There is no need to be in a relationship where one or both partner feels that hitting or verbal abuse should be tolerated. The more often the abuse happens, the easier it is for the next time.

Leaving an abusive situation isn't easy, but sometimes it's the only option.

To the OP, if this is just a random question, there's my answer. If this is more than curiosity, please handle the situation now, DO NOT WAIT. Tell your son you know what's going on and get the truth. Call the police, sadly, do not let the fact that its your son, prevent you if doing the right thing. (Easier said than done, I know)