Given the fact that shadow priests are top of the line dps coming in Legion, and his father just got killed by the legion (presumably because the horde abandoned the alliance, at least thats what the alliance and its leaders thinks happened), I think its pretty safe to say, if Anduin went up against ANY horde leader, he would destroy them.
The rage thats built up in that kid is like when Gohan turned super sayian level 2 against Cell.