Is this a new behavior? If so, I would evaluate if there may be something medically going on with him, such as a UTI.

Sometimes rabbits can get a bit more territorial if there are new scents around, so could that be happening? Have you made any adjustments to his living space recently?

Also, is he neutered?

If there’s no sign of a medical reason and he is well bonded with the other rabbit, he may simply be a rabbit who is not adherent as others. You can try removing surfaces he’d be naturally tempted to pee on and also rearranging and adding litter boxes may help.

Some bunnies are more litter trainable than others, unfortunately. It isn’t unusual for one half of a bonded pair to feel the need to mark as well.

Ruling out a urinary issue is important, and I also had a bunny with dental issues that had very bad box habits. Sometimes being in pain is enough to cause them to not care about going to the box. So it might be worth having his teeth checked if he has any other symptoms of a dental issue, esp since he’s a dwarf breed.

Sometimes it seems like bunnies perceive more of a litter box zone than a specific box, especially in bonded pairs. Getting a really big box with lower sides can help, just so they are more tempted to jump inside. I’ve also found it helps to make sure they can only reach their hay while they are actually sitting inside the box. Some of my bunnies will just side right outside the box and eat hay (and pee and poop) if given the opportunity!

Having a solid floor also helps (and at least makes clean-up easier). I use a sheet of cheap vinyl flooring in my buns’ pens. It’s slightly textured and they don’t seem to have trouble walking on it. And at least if/when they pee it’s easy to clean!