Rust is in fact the #1 concern, just from the pictures though, looks like this truck is well loved. They're pretty simple and reliable, there isn't too much that I know of with these things that you consider common issues.
The 3.0 is a reliable motor as well, I would just make sure that you get to see the truck startup cold and listen for any knocks, or ticking.
Things to check:
If it has A/C, check to make sure it's working, regardless check to see if the air comes out of every vent position. These trucks are notorious for their blend doors failing leaving you stuck with only windshield venting.
Take a close look at the accessory drive while it's running for any wobbles, nothing super vital here and most stuff can be fixed pretty quickly/cheap, but something like the main pulley wobbling could mean an annoying job in replacing the harmonic balancer.
Check the VIN/ask if the recalls have been done, I believe just recently there was another recall on the airbags.
If you have the opportunity to lift the truck, take a good close look at the frame, and check that the front suspension is in good shape.
Stuff you should know:
One thing you should be mindful of is pretty terrible fuel economy, these things are notoriously bad. My truck with the same engine and transmission does at best 24-25 MpG after some pretty comprehensive modifications, and super econo focused driving among other things I've done to get the truck to just drive more efficiently. Expect anywhere from 18-21 on the highway on a good day.
Another thing, while most trucks are unsafe compared to cars, older trucks like these Rangers are even less safe. There isn't anything catastrophic to worry about here, and I wouldn't bother to mention this, but since your son is going to be driving it, it's something you should consider. When compared to the Nissan Frontier, and Toyota Tacoma of the same year, this generation of truck ranks significantly worse, at least its better than the Chevy Colorado's from around that time.