You could put a larger litter box in there that would be much better than that tiny corner one. Something like this could work. It's 98 cents at home depot:
https://www.homedepot.com/p/Sterilite-6-Qt-Storage-Box-in-White-and-Clear-Plastic-16428960/203120094
It's still on the small side but considerably more roomy than what you have and should still fit in your current cage.
A rabbit's box should be minimally large enough for them to lay down in stretched out.
--probably the sooner the better. Since you've changed the cage, now is the time to do this. He has outgrown the current one for sure. His back end seems to barely fit.
https://www.homedepot.com/p/Sterilite-6-Qt-Storage-Box-in-White-and-Clear-Plastic-16428960/203120094
It's still on the small side but considerably more roomy than what you have and should still fit in your current cage.
A rabbit's box should be minimally large enough for them to lay down in stretched out.
--probably the sooner the better. Since you've changed the cage, now is the time to do this. He has outgrown the current one for sure. His back end seems to barely fit.