Having and outside blind cat and an outside blind dog are completely different things. Dogs aren't likely to wander off, or wander away from the yard. Cats will. They will wander off and they will get into trouble. No matter how small the number of cars in your area are he is likely to get hit.
It's best to find him a new home that is willing to cope with his disability. Hell, my cat has perfect sight yet he STILL jumped into our next door neighbor's walled off yard that had 3 PIT BULLS in it. (He probably lost 8 of his 9 lives that night.)
If you can't find a home that can take him in I agree with the previous posts that it's probably best to euth him. I feel it would be more humane.