OL Adcock Custom ACS Bows has a page on tuning longbows and recurves. His method involves a couple fletched and a couple unfletched shafts and making adjustments based on where the two groups of arrows impact the target. He doesn't address which way the nock points.
I just set a batch of arrows for each of my sons (and one for myself) using this method. My youngest is real close to your son's specs (he is even a lefty, also). Based on what I was seeing, I would guess your arrows are a little overspined. Perhaps, the tail end is slapping the rest on the way out and kicking the back end to the right.
I made my son's arrows 31" with 160gr field tips and I had to sand the shafts down to 45#. He is drawing 25.5" @ 37#. I might have too much weight in them, but they fly straight.
Good luck and give OL Adcock a good try. I will warn you that you need some lattitued in your range... I put a hole in the cabinet in my garage. Find a hill to shoot into.