Two more thoughts... If he has a 6.5 Creedmoor that he is comfortable with and used to shooting. You haven't said which rifle it is but there must surely be one in the same model available in a 308 as they generally fit in the same action. The 308 is pretty much capable of doing anything the 30-06 can with the exception of the extreme ammo options available for the 30-06. However a good 180 grain bullet will be plenty if well placed.
A second thought and I know this will get some dissenting opinions; Let him use his 6.5 Creedmoor. Us as heavy and well constructed bonded bullet you can find. Get him close with a clear target to the vitals and he can kill an eland with it. The Swedes us the 6.5x55 on moose.
On my first hunt I took a Creedmoor along. The PH loved it. However he had just recently completed a hunt with Niel Davies of Hornady testing the then newly introduced 6.5 Creedmoor. They shot a lot of game with it including eland. I believe he used 129 grain Interbond ammo and a Ruger 77 Hawkeye rifle. The biggest critter i took with mine was red hartebeest but it blew a spray of blood out the far shoulder and flopped it right there. Not ideal on eland but use a bullet that penetrates and put a hole through the vitals and the critter will die.
I would also argue for getting your son shooting from a standing position, on sticks. In fact I might argue to not have him shoot from a bench again before the hunt. All his shooting should be from practical hunting positions. Especially standing with the rifle on sticks. And loaded down ammo.
Put an animal in front of him and he won't feel any recoil so just pop in the hunting loads for the actual hunt, whichever caliber you select