Does he have a large space to move about? If so, then provide boxes, tunnels, mazes for him to explore. Having boxes with doorways cutout and creating interesting spaces and hideys will encourage exploration. The area can then be rearranged as he gets bored or used to a certain arrangement.
Just bear in mind that rabbits do tend to spend a large portion of the day lounging about. It's nice for them to have spaces, though, for them to explore when the mood strikes.
Have fun enrichment things in their area. Chew sticks, toys they can throw around, blankets they can dig at and roll up etc. I have a cat tunnel for my rabbit and she loves to run through it, and what the above commenter said.