Sometimes you have to start out with more than one litter pan, sometimes you need several, depending on the rabbit and how much space they have on their enclosure/cage. The type of litter pan is important. If it's too small or too big that can be a problem. I had to try two sizes for Sophie. Even though I thought a small kitty litter pan was big enough, she didn't prefer it, plus her rump was always sticking out the corner of it and pee was getting outside the pan.
Proper placement of the pan is important too. They prefer peeing in corners so having the pan in a corner is ideal. As the others stated, place urine soaked papertowel in the pan so they get used to that smell being in their pan.
You also have to make sure you remove all urine smell from the cage in the areas where he's peed other than his pan. Use vinegar or you can buy enzyme spray that also eliminates all pet odours.
Also, if he's not neutered, that may be making things difficult. Often when a bun that's marking territory gets fixed, they will get better with peeing....although not in every case.