It sounds like you've got the drawing issue under control.

This is the bit that I'm finding trickiest. Your DS was obviously able to read. I'm not sure if Kai can or whether he's just memorized words he needs to get by, and without sitting down and quizzing him, how will I know? He's far more comfortable on the computer than with pen and paper but again, is this something I should encourage given that he does seem to want to write?
I guess what I'm wondering is, would there be any benefit in setting aside time each day for a structured practice session for the reading and writing? Given his attitude though, I'd most likely have a battle on my hands. ((waving to you, Twinkle Toes!))
Why do you need to know whether (and to what extent) your son can read? What do you think he'd get out of structured practice time?