"GS8 writes left handed but does most large muscle activities right handed."
This is me. (and I draw pretty well, too)
The lack of coordination can come from living in a right handed world when you want to grab doorknobs, put on baseball gloves, use those stupid knobby things they put on pencils all with your other hand. frown

What you mention about the tendency for large motor right handedness may come from a sort of reversed brain/eye dominance. (Reversed from what is 'normal' but not in a bad way)
I have read about this, but can't find where...for me it means my right brain is dominant, at least a bit, so I'm left handed. But also, my right eye is dominant - there is a simple test you can try for this. So in more full-body oriented tasks my eye dominance tends to take over.
Most folks are going to be left-brained, right-eyed, right-handed. Right brained, left-handed, left-eyed.
Has anyone else read about this?
Not sure if or how it would affect drawing ability except that I have heard that left-handers tend to go for art...

Do you mean some letters are written backwards or that he is switching out a couple or more letters in a single word? I think the first scenario is not dyslexia related and not a big deal at that age. The other scenario, I am not sure.

Here is something about the eye dominance, including the test:
http://ifmomsaysok.wordpress.com/2008/06/30/right-or-left-eyes-brains-oh-my/

Last but not least...my Grandad was ambi, batted left& wrote right. I can draw with both hands, although I usually rely on old lefty. DS8, looks pretty righty all the way, dd2, shows a little ambi tendency but it's way to early to tell. DH is Lefty all the way! I can assure you that all of this is entirely normal, and yet... wink