Rail Pass in Spain

Eurail Pass is a major pain to use in Spain and neighboring France: a hard "no".

For use within Spain while he studies there, the tourist-oriented passes sold by the Spanish railways (Renfe) won't really work out, since they're meant for intensive use over a few days or weeks.

However, if your son is under 25, Renfe has a discount card. Never used it, but could be useful. And for many destinations in Spain, the bus is a very good option: cheap and often faster than the train, especially if your son is not starting from Madrid.

When your son has a long weekend, he may want to hop on a budget flight to a more-distant destination, so that's another reason why a rail pass might not make financial sense for him.

I agree that a rail pass won’t pencil out in savings.
What he can do is what the Europeans do, and buy tickets on the fast-trains in advance for the routes Madrid to/from Seville ( a big university city) Madrid to/from Barcelona, Madrid to/from Granada and many other Spanish cities. When bought in advance these discounted tickets can be less than €10.
Officially, these tickets go on sale 90-120 days in advance of the travel day but the website www.TheTrainline.com has tickets on sale now for travel through April 2023.
The French national train company, SNCF, sells discounted “Prem’s” tickets beginning 90 days in advance for travel on its fast TGV trains for as little as €19 for routes like Paris to/from Perpignan, Carcassonne to/from Paris, Nice to/from Paris. The Perpignan route is particularly handy for linking Spanish train travel to the French TGV routes because Perpignan is only an 80-minute train ride from Barcelona.
www.SNCF-connect.com has details.
For European cities further afield— Amsterdam, London, Berlin, Prague and more— finding a deal on a flight is usually the ticket to buy. www.Skyscanner.com
has a calendar showing the best days to fly to get the best prices. You can sign up for Fare Alerts that will e-mail you when fares drop. www.GoogleFlights.com also offers fare alerts to those who sign up.

As a resident, he would be eligible for the InterRail Pass. Anyone know if there's an advantage to that pass, or the same limitations as the Eurail Pass?

To qualify for an Interail Pass, the rules require the pass holder to have an official European Residency Card and also to have been a resident for at least 6 months.

This may be useful:
"A beginner's guide to train travel in Spain": https://www.seat61.com/train-travel-in-spain.htm

It might make a difference in where he is staying. The travel options for a travel hub like Madrid are different than from a remote college town like Salamanca,

Thank you all for such great input! My son will be in Seville (I forgot to mention that), and only for about 3 months. I appreciate all of your feedback!

Oh, if he is in Seville, then basically Madrid (incl. surrounding towns), Valencia, Zaragoza and Barcelona are the only viable long-distance train destinations. If he is interested in those then he should try to book tickets in advance for best fares.
Otherwise, there are plenty of places in Andalucía that are easily accessed by local trains and buses, and getting to further-flung Spanish cities or another European country (including Portugal from Lisbon to the north!) typically involves a flight.