You know, my daughter sounds kind of similar. She read anything, and still does, and could write letters but didn't really want to write much, until she started K this year at nearly 5.5. She's really blossomed as a writer, though her hand gets tired. She's learned how to really do punctuation, dialogue, a lot of spelling, etc. but going into K, eh. So I think it's not necessarily a sign of a potentially lingering weakness. The split sounds ideal, especially because then he'll have friends moving to second.