Take it apart and measure. I'd guess it's 26.4 but who knows? Even if it's 27.0 a good shop could cut it down to 26.4 if you want more headset choices. I have no trouble removing all the headset pieces with a punch and hammer but YMMV.

Welcome to BikeForums! Sounds like a nice bike and I'm sure it will be a great rider for your son. While you're at it I assume you're going to overhaul the rest of the bike, too. Oh....we'll need pics once it's done