Hm-m-m-m... well... I don't know.... (I take it perhaps this is a son?) It's possible the sexism is something that was picked up either from a relative or from friends. If the person's sexism is directed at you, then it's a matter of establishing & enforcing some personal boundaries. If it's a matter of the person making general comments you don't want to hear, one option may be to simply say you consider what they're saying to be sexist & you don't want to hear it. Tell them to please refrain from making those kinds of comments in your presence. And if they do it again simply walk away. (Again... boundaries.) Yet another option may simply be to walk away in general. Sometimes things like sexist comments can be primarily an attention getter. So if you withdraw your attention, you eliminate the potential for reward.

Also if the person's sexism is coming to light in terms of general comments that are made, or if it's in ways in which you observe the person to be treating others, another option might be to offer some alternative ways of looking at situations or alternative ways of handling situations when the opportunity presents itself. But if the person's sexism is not directed at you personally, at least from my perspective, it's not really your responsibility to modify the person's attitude or behavior.

The other possibility here, I suppose, is that the underlying cause relates to some personal insecurity regarding the person's own sexuality or self confidence. If the person feels themselves to "not measure up" in some way or another making sexist comments, or acting in sexist ways, may be the person's way of trying to make themselves feel better about themselves. I sort-of doubt there is much you can do about something like this. It's really something the person has to work through themselves in their own time, perhaps with the help of a counselor or therapist, if -&-when they come to realize what they're doing is not helpful. At least these are my thoughts with regard to your post.