Peeing outside of box.

Hi Dr Marie,
My Cat Nikki is 21 yrs old......he was shot in the mouth when he was 1.5 yrs old, and still has the bullet lodged behind his shoulder blades......he has been a very fiesty, strong, healthy cat.
He eats a great deal...combo of wet food and dry. He is now blind but amazingly manages to get around with no problems.......he does not groom as much in the past few years....since his accident he has very little of the combing texture on his tongue and now he has many burrs in his fur. I try to bbrush him, but he hates it and puts up a fight.....I jut don;t want to hurt him.
My major issue is his urine habits.....sometimes he makes the litter box but mostly he pees on the carpet.
I restrict his open areas in the house but the smell and energy it takes to clean up and change papers and plastic are becoming too much. He also pees alot....in the morning I have at least 2 or 3 areas from during the night. We clean the litter box all the time making sure it is always fresh for him.I watch him after eating and immediately take him to the box....but he'll leave without peeing or do it out of the box a few feet away.
He has used this litter box and this litter for years, we do have 2 other cats and they have all shared these items in the past (we have an additional box upstairs as well but keep him away form that area of the house now. I have avoided the Vet because I am upset about havng to put him down if that is my only option. I thought he would have gone on his own a few times, but eats and gets around really well considering the blindness and his age......please advise.
Marie