How do you get him to be himself around you?

Its like opening a rusty door it takes a while to open the door
but if the guy likes you enough. He'll let the door open himself.
the trick is to make him feel comfortable. I know I wouldn't open up much if I was nervous or didn't know what the girl would think etc. nice question.

If he can't be himself from the start you should reconsider his friendship. Loyalty starts early and unless he is very nervous around you he should open up his true self.

As well as YOU being YOURself, you might also like to do things that he likes doing, like if he likes mountain biking, for example, you could go mountain biking together. I find that I feel more comfortable and more able to talk and act freely in my own element, and once that comfort has been establish between you two, you can move on from there. Maybe something competitive like tenpin bowling might also be fun, I dunno, but a little bit of competitive banter is fun.

Just be honest with him about how you feel about the relationship, if he knows you are not playing games, he won't feel the need to play games too.

You be yourself first and he will see that you are being honest about yourself around him, which in turn will make him relaxed and make him be himself when he is around you.