you might all ready know some of this but i will just say it just incase.
1.make sure you put the litter box in the corner he likes to “use”
2. only put his hay in the litter box until he is trained.
3. don’t clean the box to much, cuz then he won’t know were to go.
4. when ever he goes out side of the box pick it up and put it in his box, if he pees wipe it up with a paper towel and put some of the pee soked paper towel in the box, then clean the spot with white vinegar.
5. clean his cage every day! and scrub the cage with vinegar but don’t clean his box every day only every few days and when you do clean the box make sure you put some poo and a small pee soaked paper tole.

i know like it sounds like a lot of work but he should catch on soon. this is just worked for me, hope it works for you!

You mentioned in your intro thread that you have kittens that like to use his box. I think this would be a big factor in his peeing out of the box. If you confine him in a cage or pen, it should help get his habits back on track. It will also prevent the kittens from using his box and if he begins to use it again, you’ll know that they were part of the problem. You may need to secure his boxes to the cage or pen sides so he doesn’t move them.

From what you’ve described, it does sound like a marking behaviour but loss of litter habits can indicate a health problem – so keep that in mind.

Can I ask a question? Why clean the cage every day? Should we do this if the bunny is peeing in the cage and not the litterbox or we should do it anyway?

Just for litter training and for when they are peeing in the cage. It helps define the toilet area from the rest.

You may want to keep Mungo temporarily on a non-absorbing surface such as a tarp to littertrain him. Do you have a pen for Mungo?