It sounds like he's got some issues to work out with you. Hopefully you can talk about this calmly and in detail when you both have some free time. You need to find out if he has legit concerns with you smoking MJ (like kids catching on, or you getting busted) or if it's a control thing.

Only when you get to the bottom of what is troubling him, can you work toward a solution. If he quickly gives in, without discussing really the problem, then you'll likely be revisiting the same problem somewhere in the future.