I can't really point you towards a store in San Antonio. When I first started gearing up, I went in one (forget which) to grab some trekking poles before a weekend trip, they were only $15 more than I could've had them online but the weekend was coming fast so I went for it. Then they tried to sell me a $250 pair of boots on the way out the door.
If Austin's not too far; call the downtown REI in Austin and see when Katie will be in. She's a member of the Austin Backpacker's Meetup Group and went to Big Bend with me in late Sept. I'm sure that she'll steer you in the right direction without trying to empty your wallet. Let me know if you have trouble finding her, I've got her phone# and can call her directly but I don't want to give it out without permission.
Otherwise, I've gotten bits and pieces from various places. Your son would probably be fine on a Blue Foam Pad from Walmart, grab some waterproof stuff sacks and para-cord too while you're there. Other ideas might be GPS, compass and book on orienteering, a platypus hoser hydration bladder. I got a Magellan nylon poncho, windproof fleece gloves or windproof fleece cap from Academy.
I agree with finallyME, look a lot before you buy and make sure that you get the lightest version of whatever you buy that will do the job. Wait till you get everything else before you buy a pack and KEEP YOUR RECEIPTS.
By the way, send some money to finallyME, it really does tick him off.