If your relationship with your dad is one of mutual respect then I would suggest that you expose the elephant in the room as it were, and let the chips fall where they may. Inform him that you feel like you've let him down {and yourself} and it's hurt your relationship...putting a doubt in his mind of how you can be responsible. If you've joked around in the past about getting high and dad seems to be ok with that....cool. Don't push the point, allow life to work, allow experience to teach and then when the time is right it'll happen.
Patience is key
Be glad you're not having to hide the fact that you and Dad smoke weed, but be cautious, not everyone would approve.....think about what your dad is having to deal with. Not only does he have to be concerned about his habit but now his son follows in his footsteps, the stigma of being a toker is hard enough to deal with in certain adult situations, let alone one where the child is involved.