Sorry for the shortness of the last reply, speed was of the essence. I am so very sorry that it's all come to this. It's possible that not being able to wee has led to kidney failure, but then maybe kidney failure may result in not being able to wee? I would be thinking that the lodged kidney stone would be the culprit of the chain of events, but not being a vet I wouldn't be sure.
It's hard at a time like this, but if having answers helps you in any way then now may be a good time to speak to the vet, along with offering to have his medical history sent to them to help them to deduce the most likely cause.
Massive massive hugs to you, having recently lost one who went downhill without anyone being able to save her, I know what a painful thing this is to go through, and it rips your heart out knowing that they cannot be treated.
It's a really awful thing to have happened and you've really been through it.
Be free at the Bridge sweet Stanley x x