Dont just let him catch you my friend or you will risk a trust/suspicion problem.
If you want him to know ( great )just sit down and tell him straight out. Any parent would respect the honesty and the courage that would take and that will build trust as opposed to killing it. Even if he's not cool with the idea he will no doubt respect that you took the initiative and shared it with him. He wont be wondering if youre honest if you prove you are before the doubts set in. It could be a building block for a great open relationship instead of the steroetypical parents vs teenagers scenerio.
I respect your balls and your desire to not be sneakin around behind his back.

Cheers!