Looking at the photo's......I was wondering if your son cleared the bed rail of the truck with his shot? Seems a bit too close for comfort to me. Sure these things are a blast to play with, but I think we all need to remember just how powerful these new toys are. Forget the damage to the truck, but the ricochet can do a whole bunch of damage as well. The photo shows a situation that is unsafe on so many levels, probably the worst is that your grandson is in front of the firing line.
I think a great way for you to get your grandson into the sport would be to time his next visit with a hunter safety course which the three of you could attend. It would add another dimension to the experience for all of you. I also think a youngsters first few sessions with firearms be at an actual range where they would become familiar with safe range practices. This would set the stage for many years of safe, healthy shooting.
Didn't mean to scold..... God knows I've done so many really stupid things in my life and will probably do a few more; but I think safety is probably the most important thing we need to learn and remember when we go out to play. Hey, maybe it would be a good idea if some of us old farts who took the safety course 40 or 50 years ago took a refresher- Just to be on the safe side!
PS- A couple of saw horses at different heights would make some great, portable rests which could be brought to the site. Of course, you'd want to take them out of the truck for use!!!LOL