The most important thing you said is that there is a change in your boyfriend’s behavior. Regardless of what that change is, the fact that his behavior IS changing is cause for concern, which you clearly have. I think that short of asking him what’s up – not where he is or where he disappears to, but what’s going on with him [i]beyond[/i] that, there’s not much you can do.
He’s aiming for more privacy and freedom, and while that can mean he’s simply busy with work or contemplating his future, it can also mean he’s looking for more freedom from his two year relationship with you and is testing the waters. He hasn’t thrown you any red flags yet, but you’re wise to notice this yellow one. Talk to him. Try and open the channel of communication, and don’t make him defensive. The truth is what you need to hear, regardless of what it is.