If he smokes it then he already knows you do... Think about it

He smokes weed
You smoke weed

He doesn't know that you know he smokes weed
but you dont know that he knows you smoke weed

Sorry if that was complicating but at the end of the day if he smokes weed then he knows all the signs to look for so you might aswell admit it to him because he already knows he will be cool about it.