If I understand you correctly, you say his dad uses a .270 of yours. Well if the lad can handle such rifles as a .243 up to a 30-30 in the recoil area, then he can handle the .270 in 130grn nos. That combo is more than a match for any cow, like any rifle you may choose, take out heart or lungs and game over. Limit his shots to whatever he can shoot consistently accurate and the gun will do its part.
Make sure he practices a plenty to build his confidence. Also make sure he' s coached at the moment it' s time to take his elk so as to relax and assure him.
elknut1