Just looking into this myself.
Top of the list so far is trek Superfly 20, 8.1kg and has proper MTB sized tyres, proper bars etc (only issue with it are cranks that are a bit long)
Next is the specialized hotrock street (lighter than the normal version as has rigid forks) just needs MTB tyres.
He’s on a frog 48 at the moment, but want more of a proper mountain bike as he gets more adventurous.
Isla have caveats that say the bikes are only for “light off road use” or something like that so not brilliant if he snaps it in half and hurts himself trying a drop…