It’s great that you rescued this bunny and are trying to improve his life! Hopefully once he’s neutered he’ll be happier. I’m sure as he gets used to you he’ll become more friendly too
Can you try putting puppy pads on the floor? You can normally buy big boxes pretty cheaply, and they make cleaning up accidental pees SO much easier!
Wick has good tips for litter training
Some ideas for playing could be hiding treats or pellets in a paper bag, or stuffing them into a toilet paper tube with some hay, so he has to work to get them out. You can also try giving him willow or apple twigs to chew on.
For a favourite snack – do you give him any fresh greens? Rabbits should get a small portion of greens every day, normally leafy ones, like romaine lettuce, parsley, dill, basil, coriander/cilantro, kale, rocket, carrot tops, celery, spring greens. If you start slowly introducing veg to his diet (just give him a small piece of one, if you haven’t given him any before), then you’ll soon find a favourite! Small pieces of apple, grape, or carrot can also be given as treats, but as they’re quite sugary it’s best to limit them.
I was also wondering if you feed him hay? Hay is a really important part of a bunny diet, and they should always have access to good quality, green grass hay 