Get him to go to your local kite shop and try on a few different harness', most models come in xs which should fit him well. I'm 14 and use the Mystic Warrior 3 xs which is a great fit, i would also look at shortening the chicken loop if possible to give him a few extra inches reach if he needs it. Also i would recommend to try steer clear of the Dakine harness range, not because of quality or any issues, just because their spreader bar is quite a bit longer than most, and will give him less reach and possible cause the harness to ride up a little more than usual.

I started on a 131cm board, already had it and at the time it was great to learn on, only found it started to get to big after i started getting into tricks, which i then moved onto a 127cm F-one Acid (would very much reconmend, has good flex for juniors and foot pads are great for small feet). Let me know if you have anymore questions

Ive got kids we use Prolimit seat and waist harnesses and my son rides a 134 board the board dosnt have to be to small
Have fun
George