Rabbits will poop the most while eating. Put the hay in or, better yet, over the litterbox and most of the poops will be inside. Pick up the poops he leaves outside of the box and put them inside. Most rabbits are never 100% for poops.
We have had 2 out of 42 rescues that used the litter box exclusively. As long as they urinate in the box they are trained--they poop here and there, and everywhere to mark territory, which led me to buy a large shop vac.