Your child's symptoms can be due to entopic phenomena. This is where you see things that are due to the makeup of your visual system rather then something out in the world. It is a normal thing.
Here is a link to a good article:
What your child may describing are either purkinje's blue arcs
or maybe blue field entoptic phenomenon, as discussed in the article.
One thing that doesn't fit, however, is that you say your child's vision is blurry, and that doesn't occur with these things.
I would suggest that you have your son examined by a pediatric ophthalmologist to make sure everything is OK.
Does this make sense to you?
Let me know if you would like to discuss this issue more. Have a good day.