If you're ok with single-shoulder carry, I love my Maxpedition Lunada. Very compact, but big enough to hold a liter Nalgene, rain coat, flashlight, multitool, granola bars, tourniquet, 2 pistol mags, extra batteries, and miscellaneous survival items. Oh, and a concealed carry pocket that's pretty accessible when you swing the bag around.

It's larger than what he needs, but not cumbersome by any means.  I can even use it for an overnight trip if I had to.  A USGI canteen fits in the bottle pouch on the side and I can use the MOLLE on the other side and an adapter to mount another 1 qt. or even a 2 qt. canteen pouch.  A pistol will fit snugly in the laptop compartment and won't have a chance to catch on anything in the other compartments and can be accessed quickly with the pack removed.  I suppose one could mount a pouch designed to conceal a pistol to the MOLLE on the side, too.

I did add some features, though, such as a hydration bladder pocket and suspension strap in the main compartment, a port for the tube, compression straps, MOLLE on the bottom, straps down the centre of the shoulder straps, and a waist belt and sternum strap, as well as the mesh material in the backside.  I like it a lot.

They sell the identical bag at Walmart for like $25 under the SOG name, black only though.  I like mine, I use it for my .308 stuff when going to the range.

I should have suggested this then

http://www.mysteryranch.com/booty-bag

I bought an EMS Women's Mojo as a cycling bag which also works really well for trail running and sort-of minimal hikes. With coupons and sales, you used to be able to do them under $30- but I believe that they are discontinued now.

Maxpedition monsoon is what I have for everyday use. Used it on a couple hikes and it works well. Not too big but lets you carry a decent amount. Would have liked a non-fixed strap so I could change the shoulder orientation, but it still works.