I think in your case the first thing you need to ask yourself is, do you want more than just a sexual experience?
If you are looking for something long-term, then the information about marriage and reproduction will obviously be important to you, and in that case, I can see why you’d want him to know it as well.
One way you can possibly let him know is by having a book lying around in your room or mentioned on one of your personal online profiles. So far, I found two decent-seeming books that address these themes directly: Forbidden Relatives: The American Myth of Cousin Marriage by Martin Ottenheimer and Cousins: A Unique and Powerful Bond by Johanna Garfield. I’m in the process of reading these two myself to learn more so I can’t say much about their contents as a review yet, but the topics are both obvious upon first glance. If you comment on either of them very generally, and place them in view of your cousin, you might be able to communicate your thoughts on the subjects and learn his thoughts without having to admit your feelings towards him specifically. You can do the same with a print-out of an article or by sharing an article on an online profile (Facebook, Tumblr, etc) – somewhere he can see it. Maybe even tag him on it.
You can say that you’re doing some research about different kinds of relationships or kinship in different cultures and you found these books or articles and they looked interesting. Then you can ask him what he thinks about these topics. You can ask it in a neutral, discussion-like tone, and if he asks you if you are trying to tell him something or asks if you are hitting on him, just act like you don’t know what he’s talking about and that you are just asking a general question, for a writing project or for school or for your own interest. Make your intentions seem as disinterested as possible. If he still acts suspicious and reserved, just stick with the generic actions of spending more time with him and seeing if he tells you his feelings on his own. No harm done.
You can also try talking about controversial subjects in general one day (cross dressing, plastic surgery, furries? .. lol) and ask what his opinions are one by one, then switch to relationship types (long-distance, relationships with large age gaps, gay relationships, interracial relationships, polyamorous, etc). Mention random interesting statistics. Somewhere in there throw in cousin marriage and some statistics on cousin-cousin reproduction, and see what he says. Make it fun. Try to hint at your own opinions without saying too much. Try to get him to express his opinions first. Don’t linger too long on that one point, and continue your conversation as though it’s all just for amusement. Maybe somewhere along the way you’ll find out what you need to know and if you and your cousin are on the same page. 🙂
Good luck! And keep us posted on your progress if you want!