Well, I am not sure if he is really asexual, or he is just not have sexual feelings for your friend.
Scientifically speaking, he is a sexual creature because he is an animal and animals reproduce sexually
But if he does not have sexual feelings, that is different. Humans were built to have sexual feelings, so maybe something was wired a little differently in his brain. I don't think you can choose to be asexual. Obviously some people have a stronger sex drive than others, but normally everyone has some sexual feelings. I'm not saying this is necessarily a bad thing. If he is happy with not having sexual feelings, that's fine