If he's on a lead, you might give a lil tug and a friendly call. Maybe bait him out with a favorite treat. Bunnies are very smart, and the best nose around. I'd have to say keep an eye on the area- he'll pop up! Just every now n then give a lil tug on the leash to let him know he's gotta get a move on... Good luck- some bunny loves you.....

 If you know the burrow then put some of his favorite food down. If all else fails its possible to smoke it out of the hole. Which could be dangerous and i would really advise doing it to a pet.

 Yeah i suggest putting food down it and also at the end of it so at least eventually he might come out completely. I really do hope you find a way that works.

Good luck