What does a viagra (sildenafil) do..how can i give it to my bf without telling him?

BAD idea: If he gets normal erections, it's not going to make him a better lover. If you administer a crushed tablet to him in his food, he may develop an intractable, painful erection that can send him to the emergency room in urinary retention. He will probably figure out that you did this, he will discard you and will be within his rights to file both civil and criminal charges against you.