So long as the iron sights are functional, I would use them to start. Reactive targets are fun, but I would get him started with paper (not too much, maybe 30 rounds or so depending on how he does) so that he can see where he is hitting and you can address whether he has the basic fundamentals down. Remember, reactive targets are only fun if he is hitting them
For targets clays are fun, various fruits and veggies are fun, but my little brother (14 as well) has really enjoyed shooting suckers. Due to their small size they might be something to get him shooting at on a 2nd or 3rd range trip, but are pretty cool when a bullet shatters them, IMO. Another easy one is if you have water bottles that have already been finished to fill them up with tap water and some food coloring. You can make the simplest things into fun targets, and I enjoy it when I think of something new and clever.
