Run the google machine about the 6.8 SPC and SPC II and you'll find untold volumes of information about the difference.
Long story short (you won't believe it, but this IS the short version, it's been a long ride, almost as bad as the 6.5 Grendel/.264LBC/blah blah debacle), the original specifications for the 6.8 SPC chamber yielded less than impressive results. The bullet jump was quite short 0.050", so heavier bullets had to be seated deeper into the case, and it all ended up causing the cartridge to run higher pressure than it really should.
So in the meantime while that was getting kicked around through SAAMI application process, someone figured out that if you lengthen the throat a bit, you can bring the pressures down and push velocities up, or bullet weights up. The SPC II chamber is 0.114" freebore, Noveske's is 0.100", and so on. It seems that the SPC II passed muster with SAAMI as well, although I'm not sure what that really means in terms of ammo supply and reloading data...
Continuing....
MOST manufacturers jumped on the SPC II pretty quickly, but not all did. Ammo manufacturers have been left in a hard spot, because there are enough SPC I chambers out there that they have to down-charge their ammo to prevent overpressuring these short-jump chambers. So at the end of the day, if he has a SPC II chamber, it's good that you'll be reloading for him, otherwise the SPC I is comparatively lackluster.
The current Stag 6.8's, according to their website at least, are using SPC II chambers, so assuming you can verify your son's rifle is a II chamber, you can load whatever level of data you find.
If you find out that it is indeed a spec I chamber, then you'll need to be careful as you sift through loading data, and keep an eye out for pressure signs as you work up loads.
(Not so different than the .223rem vs 5.56 NATO vs. .223 Wylde chamber issues, if you're familiar with that).