You can watch the big auction sight for various vintage sights. There are a number of them available that you can tape on.
Not sure that a sight will help his particular problem though. To use a sight accurately, you have to know the distance to the target. A sight isn't going to help him determine the distance to the target.
At 6 years old, it may just be that, like most of the rest of us, he just has some trouble with accuracy, which may have nothing at all to do with his eyesight. Why is he "forced" to shoot with one eye closed? I know some people choose to do it if they are opposite eye dominant. If it's just an eye dominance thing, he could also switch sides. No better time to do it than when one is young and learns quickly.
Good luck.