It’s very normal for kids this age to push back a bit against their boundaries and to try dipping their toes tentatively a bit into “the forbidden”. Without judgement, without rancor, without carrying on like the world just flew off of it’s axis and came to an end, sit down and talk to your son with only your unconditional love.
Maybe take him out for lunch to a neutral place where the eyes and ears of the entire house and community aren’t looking and listening in on his personal business. And then, listen to him. What is your son’s motivation for wanting to fool around with pot? Peer pressure? Curiosity? A need to be on equal ground with his friends? A need for escape? He likes it? Once you find the motivation then you’ll have a better idea of how to go forward. Just always, from a place of unconditional love and acceptance. Flipping out and hard words only serve to shut the conversation down and you want always that your children should seek you out rather than hide and be secretive from you.
From my own experience with four sons… I don’t think your son is hardcore. Hardcore would never leave a rolled joint laying around in his room begging to be found. It reeks for one thing, it goes stale like a pack of cigarettes opened and left laying around for a week for another, and… hardcore would have already smoked the thing. Talk. Go talk to your son like the almost man with a few things yet to learn that he is. Use the discovery of this joint to bring your relationship closer together rather than as an alarming wedge driving you apart. I will pray for you. For peace in your house and for a son to make good decisions. Best wishes.