you should just straight up go to him, with plant in hand, and say " dad, what the hell is this"
and see what he says, if he denies it. tell him you think that its marijuana, and see what he says. you can have a nice little talk.....then if hes cool with it...maybe u could show him yer indoor set-up.
but, my dad smoked, and he was the funny type of guy.....and he would of never let me grown indoors back in the day.