Tell him. You can ask him not to note it down in your records if you like. He's bound by privacy laws not to share that info with anyone else unless you authorize him to. If he's one of those judgmental, uninformed, closed-minded doctors, you'll know it by how he reacts. If he's like most of them, he'll be well aware that tons of people use cannabis and be cool with it. There are lots bigger things in medicine for them to worry about. Like substances we can't talk about here.