At age 40, I'm actually old enough to be your father but that's another story. LOL You're taking the right approach. I have three little kids (two girls). Short my daughters coming home with a shaved head or dyed pink or purple, etc., I'm going to be pretty hands off on how they wear their hair. Definitely on the curly/straight front. My hair is actually curlier than both daughters.
Do your thing with the curls. I mean what's he going to do kick you out of the house if you don't straighten it? And if he goes that far someone should kick him in the ass.