Am I wrong for being turned off by this

I've been with dating this guy for a few months and I was over his house watching TV. A boy came through the door. It was his kid. I guess he forgot his kid was coming over. He asked "Who's this?" My boyfriend said "My girlfriend" his son said "Oh is this gonna be step mom number 3?" My boyfriend said "Oh go to your room you fucking disappointment!" So his kid goes to his room, slams the door and turns on music. His son may have been rude but I think calling your child a disappointed is messed up. I told him he didn't need to say that. He yells for his kid to hear "Dont worry about him! He knows I wish I pulled out 18 years ago!" That just rubs me the wrong way. Yes his kid was rude to me, but if you that's how he treats his son and it just turns me off and makes me see him differently. My mom said I should be happy he defended me, but idk.. Opinions?