Advice for litter training

We are having a little trouble with litter training our latest foster, which is a bit of a problem as he is a house bunny and we rent, so we need him to be fairly well trained before we can let him have more space to run around. He's clearly desperate to be able to go loose (keeps making a racket on the pen!) so its hard not letting him out, and we have read it is best to get him trained in a confined area first and then increase his play zone?

It's proving difficult as he isn't used to eating much hay so he wont instinctively poop and wee as he eats his hay in the litter tray. He also drinks lots so his wees are absolutely huge, which is really horrible to clean up all the time! Plus when its not on the tray it gets on his feet and then the whole pen is all wet with wee.

We thought he might be trying to mark his territory if he could smell our previous female buns so we gave everything a really good clean but that doesn't seem to have helped. We are doing the usual things, like putting the wee rags into the litter tray, but does anyone have any more advice that we might not have read online? He's 3 years old and presume that the rescue center have already had him neutered.

He's such a lovely bun but is he keeps weeing like this I don't think we will be able to keep him as a house bun He'd be perfect if he was trained and ate well! Worrying is the worst...

It's really difficult when you get a rabbit like 3 years old and they have never been litter trained. They may not even have been kept very clean and most people I speak to don't realise you can litter train bunnies. A confined space is best so the rabbit hopefully gets used to going in the same place and returns to that spot but whether this will work or not I don't know. It would probably take time too. Sorry I can't be of more help. I have some buns who you just can't get litter trained. It's best to start them when young.