Lennie, who knew that Wikipedia would have sizing charts?
http://en.wikipedia.org/wiki/US_standar ... s.27_sizesThey say size 6 is a 23 inch chest, which would be a size small for the pattern. I would look at the pattern, and see how much ease there is for a small, and then decide if you should sew a small or a medium.
I checked Penney's size chart, and they say a size 6 is 23-24 inch chest, and walmart says 25.
I would say a size 6 is probably 23. Is the boy slim or husky? That will tell you a lot.