To be honest, I think you've misjudged here OP.
Admittedly I'm coming to this from a male perspective, but I don't see the comment as misogynist. In fact, your son's comment wasn't about the girl at all, but about his friends reaction to her. If he'd implied that she'd done something to cause that reaction, that would be misogynist, but he didn't. Instead, he was taking the piss out of his friend that the mere existence of a pretty girl in the vicinity causes him to have a physical reaction.
And really, the underlying reason for the comment wasn't the girl, and probably wasn't his friend either.
Teenage boys are on a hair trigger. The average male teenager apparently gets around 4 erections an hour while awake. And when you are that teenage boy, it's confusing, frustrating, mortifying. It happens when you're talking about a pretty girl, it happens when you see a pretty girl, it happens when you so much as think about a pretty girl. Hell, it happens when there's no pretty girls in the vicinity. It happens because you've just adjusted yourself, it happens because you're sat on the bus and the engine vibration is making things vibrate a little. It happens because someone on the TV just said "boobs". And most confusingly, it happens for absolutely no bloody reason at all, at inopportune moments.
And so we joke about it with our friends, because nobody tells us this is going to happen. Oh, sex education mentions that we'll get erections, but they don't bring up the sheer frequency of them. So we take the piss out of our friends, and bring up the subject, dancing around the reason why we're bringing it up, because we're trying to get someone to say "Yes, it's perfectly normal to be getting them pretty much constantly", without revealing it ourselves, in case we're the only one its happening to.
Your son said "How many times did your dick go hard when she was talking to you." He meant "My body is going fucking nuts, is yours doing the same?"