I've never studied the psychology of education; I've never worked as a teacher. Likewise, I cannot speak to any organic issues your son may experience mentally, with the number six issue. But here are some general comments.
The more senses involved, the sooner we learn things.
We need to hear math. We need to speak math. We need to see math. We need to write math.
Different areas of the brain get activated, with each sense. The brain will quickly grow new connections between these areas, as soon as it recognizes that the encoded information is related. Once these extra connections are in place, activating one area (hearing, for example) immediately activates all interconnected areas.
Make sure you engage all of your son's senses.
Also, there is a significant difference between learning (1) the English words, (2) the numerals, and (3) the concepts. Studies show that preschoolers and kindergartners who learn not only to count out loud (by repeating the string of English words in order) but also to associate with those words both numerals and visual sets of objects (that is, the concept of oneness, twoness, threeness, etc.) do much better in mathematics later on.
Make sure your son is not just counting fingers. He needs to see sets of objects, along with the corresponding numerals, while hearing/speaking the names. You can use things like groups of pennies or sets of pencils.
There are lots of resources available online.
Google keywords
images kindergarten number sets for printable worksheets with drawn sets of objects.
Google keywords
youtube kindergarten counting sets for short videos designed by educators for learning to count to ten.
If the issue with number six continues, please contact your son's teacher for a conference. Thank you for being an active participant in your son's education.
[video=youtube;xewMVtMk14Q]https://www.youtube.com/watch?v=xewMVtMk14Q[/video]