This is just an off-the-wall, off-the-top-of-my-head, and other similar off-expressions... but what if you did a 180 on this? Policing him is so icky (almost as icky as his behavior ).
What he is doing has such a flavor of "adolescent rebellion" about it. "She tells me not to do this? Who does she think she is? I'll do what I want, and mommy can't stop me," or something equally creepy.
How do you think he would respond if you said something like, "I've established to my satisfaction that you are hiding some internet behavior from me. I'm not your mother and you don't need to rebel. You're a grown man and I have no desire to check every day to make sure you've cleaned your room and eaten your vegetables. From now on, I wash my hands of your online activity. Know that my saying this is an indication that emotionally I am one more step removed from this relationship. I don't want to withdraw, but you have given me no choice. These are the consequences of your behavior. If you want different results, then change your behavior. And do it in such a way that I can see the changes."
His hiding and your checking up on him sets up such an adversarial structure. I'm not sure you can get to a good place from there. Just a thought.
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