I added a warning, for sensitive readers.
I’m very sorry that you are going through this.
His bonded mate knows something isnt right, and with the very keen sense of smell that rabbits have, I’d not be surprised if she identifies the lump as something abnormal. Cancer cells have a genome which is very different from the healthy cells of the affected individual. Either way, she knows something is wrong. It ‘s not rare for illness in one rabbit to cause behavioral changes, such as aggression in the companion bun.
You may have to separate the two, but if you do, you should make sure they can see and hear and smell each other. This will be a comfort for the sick bun.
If the tumor is affecting the spine and puts pressure on the spinal cord, it could very well be the cause of his paresis.
The website Disabled Rabbits has tips on how to make life more comfortable for disabled rabbits. http://www.disabledrabbits.com/
If you suspect that your bun is in pain, pain relief is of the essence. Other than that, theres not a lot you can do exept keep him company so he isnt lonely and scared. Rabbits are, as you well know, social creatures. Since you cant really spoil him with treats, perhaps pets and headrubs would give him some comfort.