If there really that religious bring up some bible quotes about how god put everything here for us. Weed being illegal is a law made by man and is fallible to mistake. But in case there not that religious.. If you got good grades bring that up. Keeping out of trouble is another. Any current family members who smoke like an Aunt or Uncle that can help talk to him?

Think about telling him before your mom tells you to tell him. Bring that up, that you decided to tell him without waiting on your mom for what to do. Your 18 and at some point he's gotta see you as a man. This could be a good time to show him your not scared to step up to the plate with him on anything. And before you confront him, make a list of famous potheads and there accomplishments to bring up as a selling point that where not all "dumb and lazy". Hope I could somewhat help you and good luck.