It doesn't matter what reason the child gives - hitting and choking is not allowed. Anyone who hears him say "My father does it to his girlfriend" should say "He shouldn't and you are not allowed to do that here." His teachers should be addressing this at school and you should be addressing it at home.
When you visit with friends who have children or family members who have children - does this child hit and choke those children too or is it just at school? Anytime he hurts or tries to hurt another child, your response should be strong and stern. "You do not hit! You do not hurt other children." You should sit and play with him and the other children to observe what it is that sets the child off. Give him other ways to cope with his frustration. "Use your words, not your hands". "We keep our hands to ourselves!"
I'm assuming it is something that sets him off - does he do this casually or when he's angry and frustrated? If it's when he's angry and frustrated, give him strategies to deal with that. "We all have to wait out turn, there's nothing wrong with waiting a bit. Let's play with that toy over there while we wait our turn for this toy."