Then that certainly changes the situation. He probably needs shafts/grips that are proper length/thickness at the very least.
As to shaft flex... I still lean toward lighter versus stiffer, but you'd be the better judge of what he needs regarding current flex, how fast he swings, and how soon he might be creeping into the area of needing a firmer shaft flex.
At this stage there is quite a bit of guesstimating, as he could very well need different shafts in just another year or two, if not sooner. Kids hit their growth spurts at different stages, which makes it incredibly difficult to figure out.