I also realized recently that he knows his letters in terms of what sounds each letter makes. He just can't come up with the names of the letters when asked to identify them. He can usually identify them if given multiple choice options, though. Interesting, very interesting. Maybe this is also why he can't count past 4 yet? I mean, I think he knows numbers in the sense of recognizing that 9 is more than 5, say, but when he counts things he still asks, "what's next?" when he gets to 5 every time. So maybe there is something a bit idiosyncratic about his ability to verbalize names of symbols.