Either the Maxxis Crossmarks, Michelin Dry2, or Hutchinson Cobra/Cougar combo I’ve been running lately.
The Michelin Dry2’s are awesomely fast, and grippy for what they are though perhaps not as grippy up front as the other suggestions. The name “Dry2” puts UK buyers off them, hence they can be bought very cheaply in the UK.
Crossmarks are ace, just been trying a 2.25 60a compound on the back of my bike and it’s much grippier than it looks like it should be. Not quite as fast rolling as the Michelin’s or the near semi slick Hutchinson Cobra, but fast enough.
The Hutchinson’s are way better than they look like they should be too. Proper UST case here in the case of my tyres yet they’re still only about 600g each. Super fast rear tyre combined with a front which is slightly more agressive helps with confidence and grip etc. Again way grippier than they look. But not cheap!
Whatever he gets, proper UST casings are pretty much a must if riding rocky terrain, as is some sealant. Shame his bike wouldn’t fit the 2.3″ Michelin Dry2’s in as they have a heavily reinforced casing (yet still only weigh 700g each) that seals tubeless very well, a slightly softer grippier compound, are a good size yet still very fast, and are silly cheap from On One right now!