Can I run a dual crankset with a chain guide tensioner?
This is a common setup for many freeride bikes, as it allows uphill pedaling and a bash guard.  However only certain models of chain guide support this, and the one your son has doesn't officially (though it wouldn't hurt to try if you have a crankset to test; it might affect the derailleur ...).  The DRS is eThirteen's 'official' dual chain guide.