There would be a number of places I could point towards, but these would be in Germany, where I live.
However, yes, I guess that a sexist opinion might come from earlier traumata, for example.
So, if this was the case, then yes, I'd advice for a therapy. Beware the costs, however.
Since I've begun looking at persons *individually* , I've found out that many things just don't fit in into individual persons. If you hold a cliché over a person, then it's most likely wrong, because clichés are almost *always* wrong when looking at individual persons.
The herd, however, is a different thing.
But even cows can be dangerous when it comes to defend their children, a behaviour few people have in mind when approaching them.
In my opinion, it might be better for you if you'd find a male therapist, because a talk between men is something different than the talk between a man and a women, even on a therapy-only relationship. A man could rebuke your views, so to say, better, because he's a man. My guess is that men trust each other more than a man and a woman - except within real love relationships - the same as women trust other women more than a woman and a man - except within real love relationships. It's a bit like members of a football team trust rather each other than members of another football team. At least just before a match.
Instead, I'd rather send your son to a female counselor, if that's possible. I think it's important that he learns the point of view of women as early as possible. But that's just my personal opinion.
Since I do not know where you live, however, I'm unable to give you better advices.
As I wrote above, if you were living in Germany, I'd be able to point you and your son to several places.
Edit : And be carefuil with all this gangsta rap. It's quite sexistic these days. At least talk with him about *why* these songs are so much sexist (because the rappers want to display themselves as a cliché of manliness). Seriously analysing these songs could give you an insight on how clichés of manliness function these days, too.