You should move your computer next to your door. Not to block it, but make it so he could talk to you without going all the way into your room. Or at least, if he does come all the way in, it makes it more obvious that he's checking you out for the green.

OR

When you AREN'T high and he comes in, just be like, "It's okay, Dad, I'm not stoned right now. You don't have to pretend to care about my computer."