This took me a while also.
Break inside his house: if he is in front of his house use the door on his rooftop other wise he will suspect what you are there to do.
find a cooking pot: (this is where the trick is) it's called Balyn's Cooking Pot. put the poison here.
sometimes when you need to give ppl stuff without them knowing just look for something with their name on it and put it there.
Hope that helps.