To clarify, are you asking if he needs a litterbox when he is outside?

It wouldn’t hurt, but my assumption is you wouldn’t mind if he pooped and peed in the outdoor area. If he is litterbox trained, he might feel more comfortable or would more readily use the bathroom outside if the litterbox was there. If it wasn’t there, he might hold it in, but eventually he would probably start pooping and peeing since rabbits do it so frequently. Whether that negatively impacts his litterbox adherence when he’s inside, I don’t know.

If dirt box is a dig box or something, I would always encourage a rabbit to have access to engaging and fun activities, regardless if indoors or outdoors.