That is really warming.
First off, I think that is beyond wonderful your son can see these fish!! Even if he has blindness as you say, it's just so nice he can at least see some beauty in the world. And I'm glad a Betta is one of these things. I bet they will always stay with him.
And I am glad you and your son are part of fishkeeping together. It is such a good way to bond with your family. And the fish! It's neither here nor there really, but my first memory I can think of, is me being a very very little kid, and my mom would pick me up, let me feed these fish in an aquarium, and take me off to bed. So yeah. I will always remember my Mom and fish as my first ever memory. (I've asked my Mom what kind of tank this was in my memory, and she seemed blown away I could have a memory from such a little guy. Apparently this was a semi-aggressive tropical tank, from what she recalls)
But anyways, reading this just brought a smile to my face, and compelled me to share all that!!