Do you think my boyfriend has something to hide because he changed his passcode after I went through his phone.

It could be because he has something to hide or it could be because you violated his privacy even if he had nothing to hide. Some people are inherently more private than others. That doesn't make them guilty. But sometimes they are. There's no real way to tell.

Does anything other than the phone lockout cause you to worry? If it's just that then I think you need to work on your trust issues. If other things have you worked you need to talk about them with him.

Communication can be difficult. But once you get into the habit it can be very powerful.