As general-purpose move tutors don't exist in Sun and Moon (nor does any way of getting Drilbur or Excadrill for that matter, other than hatching one), the only way Excadrill can learn the move is if it already learned it in a previous game and was then sent forward, via Pokemon Bank.
The highest-power Steel attack it has access to from those games is actually Smart Strike, from TM67. It has lower power than Iron Head, and of course no flinch chance, but at least it's higher powered than Metal Claw, and lacks that slight chance to miss (in fact, it shakes off any chance to miss, including evasion boosts or Brightpowder).