It shouldn't be too difficult but it will take time to go through the steps.
First, is the cage located where he is allowed out to play? Do you just open the cage door to allow him out? He should not be "taken out," of the cage but simply have his cage door opened. Whether or not he comes out at that time is up to him.
There should also be an x-pen or similar to limit his roaming. He needs a small space to start out. Then it is super simple for him to see where his cage (and litter box) are. Rabbits can 'hold it' for quite a while. As long as he ventures out without peeing outside the cage, he's doing great. You may not see him return to the litter box to use it, but if he isn't peeing outside the cage, then all is good.
If he continues with those good potty habits for several days or a week (each day having plenty of time with the cage door open), then the area can be expanded gradually. Each time his area is enlarged and if he continues to not pee outside the cage, he's doing great. It means that he knows not to potty anywhere but the litter box.
Don't worry if you aren't seeing him hop back in. My rabbits always had their cage door open all day long. One would lounge all day outside the cage (literally hours) and never return to the cage to potty. She only pottied in the litter box though. That's being litter box trained - not pottying outside the box. Don't worry. If they have to potty, they know where to go.
The attached photo shows a sample of an x-pen being used as transition before they are allowed more space to roam.
