SA has 2 Sportsman's Warehouses. When I lived there (graduated from UTSA) I would go to Bass Pro or Cabelas to "look" and then go to Sportsman's to buy. They were almost always cheaper. In fact, I do that here in Utah as well.
Anyways, here is my experience with 16 year olds backpacking. When I was 16 (and in SA by the way) my scout troop got a chance to go to Philmont for a week. This was a little while ago, so we all started out with 50 lb packs. However, there were 2 boys that had a hard time with the load, and one of the leaders as well. 4 of us that were in sports kept taking stuff from their packs so they would just walk faster. I started the trek at 50 lbs and came back with a 60-65 lb pack.
Fast forward to this last summer. I took a small group of scouts to hike 50 miles of the highest terrain in Utah. All were between 14 and 15. I had to keep putting weight in their packs so they would slow down.
Moral of the story, try and get him as light as you can, but don't worry about it. He will be fine. If you do go hiking with him, make sure he carries more weight than you. You will both have a better time. You will have an even better time if he doesn't know that you are lighter.
By the way, look at Enchanted Rock for a good beginner spot. You will only be able to camp 1-2 miles max from your car, but it will help with sorting out your gear, and be a great outdoor experience. Don't forget to check out the caves.