I actually break reading down into several milestones. For example, my son could sound out CVC type words at age 2, so I mark that as when he "learned to read" with the qualification that he was able to sound out simple words. In your case, I'd do the same except that your qualification is that he can sight read simple words.
So the next step for my son was to sound out somewhat harder words and also to learn some sight words.
Then there's reading fluently on a first grade level, on a second grade level, and so forth all the way up to an adult level (and beyond that, a college level).
