At Trail This we stock a Kona Lana'i would be an ideal starter bike, But you can usually find an older model with a great price at a kona dealer or find a used one for a good price. If looking at used id say look for a Fire Mountain or Cinder Cone. At 5'10 he's gonna be on a medium or a 17-18.
Other options are specialized stump jumpers or rock hoppers.