Like Leah, I'm wondering if it would be alright for him to shoot under supervision. Has his brother already let him shoot the BB gun? There are ways to make it safer and easier for him. The brass BBs, getting (or making) brightly colored large targets, and only let him shoot at the large targets against a solid backgroud, such as the side of a barn or a shed. Most injuries cause by carelessness (or sometimes mean-ness) are minor. My brother shot me in the leg with a BB gun from about 5 feet away (we'd been shooting at a target that was about 15 feet away) and it just bruised a little.
I believe that if you teach your children limitations, they limit themselves and never work out other ways to do something. Modification is key - your son has visual limitations, but if you can find a way for him to still enjoy some of life's adventures, such as shooting, it will help keep him from being resentful. (My idea for letting him drive is see if you can find a go-kart track that has bright colors marking the edge of the track. It's not a car, but at least it's something, and it's fairly safe. A friend of mine did this and loved it!)
I've had friends with physical or mental handicaps, and I have been inspired by the parents and the friends themselves for finding ways to experience things that they might otherwise miss out on if they'd said, "I can't do that because I'm ___." Instead they said, "I want to do this! How can I do this while taking my handicap into consideration?"