^^^ I would actually keep him like that for now but ensure he is using his right hand as the the driver for the stroke and that his low left hand is just along for the ride - you never want to have the arms crossed! Then in a few months or whatever he can remove the left and just slide down the grip.
I tweet - @ashtennis guru (no spaces)
I Shoot - www.flickr.com/photos/ashtennis guru/ (again no spaces! grrr)