Need advice to maximize Spanish Language experience

Don't know your son's school set up. Is he on an official study abroad program set up through his US university or on a free lance program? Might made a difference with resources available.

When our son spent a semester at the Univ of Madrid, the university had a program that paired local students who wanted to improve their English skills with American students. They met each day for at least an hour and often more at a coffee shop, bar, the park, on campus, etc -- whatever was convenient -- just to have conversation. The rule was the Am spoke only Spanish and the local had to respond in English and the reverse. And on several weekends the local students, who tended to know each other. organized parties where there would be a group of maybe 15 to 20 students. Same rule applied. He love it. Very quickly advance his Spanish skills. Also some side benefits of a quick and heavy introduction into the universities' local student culture. Also the university set up unpaid, appropriate internships with local business. That was equally beneficial. Sometimes on the weekends he and his roommate would take local trips to some of the minor tourist sites. That forced them to interact with ticket takers, restaurants, etc. and got them out of Madrid where English was commonly spoken. However, he has to work at it. Cannot be passive.

Obviously, the current situation would impact this program but he should check with his university to see what may be offered.

something like this https://www.meetup.com/Aventuras-y-deportes-Salamanca/
Does he have any hobbies? If he is there as a student, why hasn't he put a card on the student union board? Talked to the student Union? As a 'foreign' student, there should be an office for them as well. Keep in mind that any language learned as a 2nd language will be more formal than the colloquial language spoken, so he shouldn't get too smug about his ability to be understood.
So, does he only eat at the cafeteria, or does he have opportunities to shop? Use that for niceties, but also maybe getting to know someone. Or start to hang out at a local cafe place. I guess in some ways it might be harder in a university city because there will be a lot of 'foreign' students and it might take a while....or a long walk from the university area....to find a real local neighbourhood.
Any move takes some time, and part of growing up is learning resourcefulness. It will be a little tougher in the covid world, but it can be done.

I am surprised that the school doesn’t enforce a Spanish only requirement as my kids’ schools did on study abroad semesters.
There have to be locals who with their families would like to include him in their activities. Maybe dinner at a local home once a week?
Or living with a local family?

His everyday activities outside of school should provide plenty of opportunities, like going to stores and places he goes to visit. From those initial interactions, it should be possible to find a small group of locals with common interests he could spend some time with. It’s possible he could do some volunteer type work, e.g., at a food bank, Red Cross, or at a park or attraction that would put him in contact with locals. If he’s an introvert he may have to go out of his comfort zone to meet people. Years ago when I was stationed in Spain with the military, I practiced by talking with people I met while on my nightly walks, whether they were shop owners, teenagers out with their friends, and even the Spanish civil war widows who just hung out by their front doors each night, and of course, our landlord who didn’t speak English. Since my wife and I made our nightly walk around town, people recognized us. What was really nice was, no matter how much I butchered the language, I was never laughed at and people were more than helpful in correcting me.

Or join a sports club, play soccer, etc. It sounds like he has to take the initiative. Can he ask to be placed with a local family instead of staying with these non Spanish speaking students? My son had a fabulous experience living with a local family. It greatly enriched his experience.
Perhaps his college or university can help arrange this.

Thank you all for the many good ideas. I have forwarded every message to him and we are both grateful for the time you all took to reply! Many blessings, Frankie