He's not that tall (183cm) and 68/69kg as a MTBers, if he was going to do more road cycling he'd probably loose some upper body muscle mass.
I've always felt like the modern MTB sport with all those short, explosive climbs isn't that ideal for him, he'd benefit from longer climbs.
Most coaches in road cycling would be shocked to see him training, the guy doesn't even train with a bike computer and only on feeling. Last year was the first time he tried to work with a coach and more methodical training and according to him that was a mistake.