Yep, I agree - brazen it out. You don't know for sure that he knows or has proof, so you might be outing yourself for no reason.
Is there any way at all he could have found out? If you've been careful, and not shown your face anywhere you could easily call his bluff. Say nothing and he won't be able to pin anything on you.
If you panic and blurt it out, you'll be playing right into his hands. You can never go back on it either. If I was you, I'd sit tight, and pretend the link he sent has absolutely no significance. Just wait and see what happens before you do anything else.