Hello there!  Thanks for the great question!  There are lots of packs out there that might be suitable for this purpose, some specifically designed for climbing and others that might be more multipurpose.  One that came to mind was the Patagonia Cragsmith . 

What is great about this bag is the way it opens, allowing access to all of your gear. In addition it hits some of the points you made in terms of carrying additional gear. For example, there is a stash pocket for a water bottle, it is hydration bladder compatible, and has extra gear loops and a daisy chain to attach that helmet or whatever else he might need to bring along! There was a really great review left behind that is worth a read too!

Here is an example of a top loading pack. Just a different design to give some perspective. 
Black Diamond Crag Pack 

Here is a great article too about making the transition from Gym to Crag. It will provide some insight on gear that you might find helpful along with some links to other useful articles! 
Learning to Climb Outdoors: Gym to Crag 

Hope you find the perfect gift!

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