I have attended the national 4H championship at the NRA Whittington center in Raton, NM. I mention it not to brag , but that I understand the issues, especially with wanting your son to get into shooting sports, the unknown level of commitment when he's 13 years old, and that his body is going to change a lot through adolescence.
The thing about shotgun shooting is that it is very personal. People can spend a lot of time and money modifying a shotgun so that when it is shouldered, the bead consistently lines up with the pupil of the shooter's eye. Having said that, concerning equipment at the 4H championship, you literally see everything on the spectrum, from a Rem 870 or Mossberg 500 to a Perazzi or Krieghoff. Expensive shotguns alone don't necessarily translate to higher scores.
I'd like something that could be used for other clays or hunting, assuming a more specialized gun isn't needed.
This is absolutely works at the "I'm trying to figure out if I like this" level of interest. A sporting shotgun works at this level. This can include everything from backyard trap to bird hunting to the 4H shooting program. There are always trade-offs when it comes to shotgun configuration. IMHO, trap, skeet, 5-stand, sporting clays, hunting, and all other games with slightly different wrinkles - all of them have "optimal" configurations. At this point it seems to be about gauging interest. Once he has shot a while, if there is a particular game he's really into, a specific shotgun for the discipline can help, but I wouldn't start out with a trap specific shotgun, for instance. You would want him to pile up some "25 straight" patches before dropping serious money on a 36" unsingle.
Some of the bigger issues are if he can handle the weight of a full size shotgun and how much recoil he can tolerate. This is a tough call. I have seen kids have a terrible time who struggle to lift and swing shotguns. On the other end of the spectrum, I have seen kids who have a terrible time because the gun was light enough to swing, but the recoil was too much. This is one area where I wish we could suspend the laws of physics, if only for a moment, but it takes knowing your son to make the decisions.
Application matters too. A lot of bird hunting is usually a lot of walking and shooting one box of shells for a couple times a year. That is a lot different from shooting 100 targets every week.
For left handers, a low cost option would be a Browning BPS, since it ejects out of the bottom. Most right handed autos would work well enough at this stage and relatively easy to sell if he does not like it. A left handed auto is going to be a bear to sell if his interests change. Sometimes, it can be difficult to get autos to cycle light loads. I really like O/U's for hunting and a variety of clay games. They are easy to operate left handed; they usually have a variety of chokes to do several things; they are super safe - "keep the gun broken open until you are ready to fire", but they are heavier.
OP didn't mention it, but the other issue could be gauge. Since he's an adolescent, you would normally gravitate to 20ga. However, if he would seriously want to compete, 20ga is at a disadvantage to 12ga. Again, this is why it is hard to buy a gun for an adolescent.
I have seen a 12 year boy old pull the trigger literally twice, feel the recoil, cry, and then not want to touch the gun. He would not even try it again. The next year, when he was 13, he still would not even try it again due to the bad experience from the previous year. By the time he was 14 (and had grown about a foot), he picked it up and shot with us - no problem at all.
Do you think life expectancy of the gun should be a factor in deciding between auto vs O/U?
No. Which one fits him better? Which one does he shoot better? It is doubtful that he would wear out either a Browning or Beretta auto out at this age. Also, don't think that an equivalent O/U is indestructible. It's a machine. If it is shot enough, it will need springs, hammers, ejectors, etc.
I think the biggest thing is to not pressure yourself into thinking that you have to buy him one shotgun that is optimal at 13 years old and will be optimal when he's 18 and will be optimal for everything he may want to do with it.
