My son has start doing trail runs with friends and I’d like to get him a running backpack. He doesn’t want one since no one else uses them, so it needs to be something he’ll feel okay about using and it works well. I suspect anything from osprey or Solomon would be fine, but I’d appreciate any suggestions, including how big it should be. Or if there is anything to avoid.

While not in ultra territory they have hit 12-15 miles with a lot of uphill.

TIA.