Thank you from rescuing this poor little guy from being killed!
Don't know what his kidney problems are, but that could be contributing to his peeing, and that may be the reason he was dumped.
He is 8 years old, he can't see so well, has been through the stress of the shelter and is now in a new home. He is probably overwhelmed and nervous.
I would try confining him to one small area - maybe bathroom - and taking him out regularly (as you would with a puppy) and praise him for going outside.
Getting him unto a schedule will make him more relaxed.