Hi H.,
1) I don't know what kind of computer you have or the specifics of your son's vision issues... but anyway, per Kindergarten and per my kids when they were that age... they and the school used "Kid Pix" (it is a software. Google search it, or you can buy it at an Apple store).
They also have computer class at school.
And yes, they or at least my kids, know how to use a computer. At school they use Apple computers... because, it is very easy to use and very intuitive.
2) There is also:
"Jumpstart"
Just do a Google search on it. I know lots of parents that use this.
It is educational "games" for the computer.
3) ALSO:
(this is good and free)
http://www.bbc.co.uk/schools/typing/
It teaches children how to "type" and it is free and was suggested by my daughter's teacher.
But yes, a child does have to learn how to use a computer and/or type.
Because, it is not only for tests... but for their general knowledge. Learning is via computers, in school too, and per their assignments, overall.
At home, well my youngest is now 6, and he before then, could use a computer. We're a tech family anyway, but my Hubby even taught him to make stop action movies on our Apple computer. Kids can learn anything. Computers are not necessarily "bad." It is about what... you teach them... and the fun of it. As I said, my son and even my daughter... learned how to use various applications and software on our computer. Even making movies.... by filming their own ideas for a "movie" and then we taught them the movie making software, and they can make those stop action movies. This is also "educational." For example.
But again, my opinion is... that kids do not learn computers ONLY for test taking. It is an overall, learning and a vehicle for learning. And it is NOT just a "passive" activity. It is and can be, very interactive. And it is also, how a parent engages their child, on the computer and teaches them... things as well, WHILE using the computer. Therefore, it is not a "passive" activity that they do alone. My son for example, also LOVES to do "research" on the computer to look up insects and dinosaurs and sea creatures. Which he loves. We do it WITH him.
4) Here are some other suggestions (all free online):
http://www.abc.net.au/countusin/games.htm
http://www.literacycenter.net/lessonview_en.php
http://pbskids.org/lions/games/
http://www.bbc.co.uk/schools/laac/numbers/chi.shtml