Maybe your not making him feel comfortable enough to let him know he can trust you. Ask him questions you know he won't reply with an "I don't know", or any other 1 word answer. Give him somthing to reply to. There is a good and intresting person in evry1, but somtimes they need help to find their way out of their shell.
I can tell that my friend is hiding who he actually is because he is afraid that people won't like him.. How do I get him to "remove the mask" and just be himself?!
Why would you want to do that? Sometimes people are just unlikable and they sometimes don't mean to be, so if someone is hiding who they are maybe its better for everyone that they are doing so. It may be best to let him take his time, eventually he will open up. Sometimes people do what they have to do to get friends but nobody can keep up a fake personality for ever. Good luck to your friend and you as well hope this helps