See if there's a spot where he's doing most of his poops/pee, that's where you should put the litter tray. Once you've got it there, any poops that he does outside of it, pick them up as soon as you can and put them in the tray and as soon as he pees outside of it, wipe it up with a paper towel and put that in the tray as well. If after a little bit of time you're still having trouble, it may help to reduce the space you have available to him so it's easier for him to get the hang of it. You may have trouble with the poops at this stage because as you said he'll be doing a lot of territorial marking to get his scent all over everything
Also, put a handful of hay into his litter tray as well, rabbits like to have something to munch on while they're on the toilet (for some reason, lol).
He def is territorial marking. I do have 3 others but are in their own room. My 2 bonded dwarfs are both fixed. My holland is not fixed. I thought about getting him fixed but he's 3 and he doesn't seem to be a brat. So I will be gettin Gunner fixed. Cause most likely he'll be free range. He seems to be trying to use the box, but he's not quite figured it out. With hard work I'm sure in a few weeks he'll be using it (crosses fingers)
i personally wouldn't give a non-toilet trained rabbit free run of anything. Like a toddler they need to earn their freedom. gradually increase the space they are allowed out in. Best and easiest way to toilet train them.
I had a Flemish mix and he was pretty much the best thing ever
If he's marking in a corner other than his potty, you can blot it up on a paper towel and put it in his box. Clean the area he pee'd on with a vinegar/water solution to keep his scent off of it. Poops you can scoop up and relocate. Hay in the litter box is definitely helpful.
Something that was challenging for me with my boy was to find toys and accessories that came in extra large. I bought this for him to use. Not sure how big he's going to get but it's worth a look!
My best advice is to neuter asap. I picked up a rescue from a pet store, so I am not sure how old he was. He was still quite young but old enough to neuter. He went straight from the pet store to the vet. He has perfect litter habits, naturally (meaning I didn't do anything special to train him.)
For contrast, my older bunny wasn't neutered until he was years old. Because of him, I still have to vacuum and sweep every single day.
I can tell whose is whose because the older bunny is a 2 pound dwarf and the younger bunny is pushing 12 pounds.
The next advice I can give is that they like to poop while they eat, so place their food in such a way that they have to sit in litter while they eat.
I'll have to buy a bigger litter pan. He does not poop on my furniture or anything, he does mark up the comforter I put down for him and pees on it. I have to wash it every day lol Gunner will be 10 weeks nov 11th so I'm not sure he's ready to get neutered. I do plan on it tho. I have 2 dwarfs and a holland ( he's not fixed)