Hi, I would advise he find a humble bar where none of the locals speak or understand English. The likelihood is that if he visits a bar where some of the patrons or staff speak some English they'll want to practice their English — and he won't get much further on with his Spanish.
I know what I speak of because that was precisely my experience when moving here (Catalunya) in 2003. My most local bar, great place, wonderful people and staff, has a good many customers who can speak English (mostly professionals) and they wanted to practice their English. So, I deliberately scouted around the neighborhood to find a bar or two where NO-ONE could speak or understand spoken English. I was obliged to communicate wholly in Spanish (y una mica de català). It rocket-fuelled my learning and understanding of colloquial, everyday Spanish, and I made some good friends and acquaitances in the process. Spanish as is taught at school is a very different beast to the Spanish spoken on the street. He just needs a bit of confidence and get out there.
Another strategy, if your son enjoys watching and/or participating in sports, is to join a local sports club. For example I am an abonat — season ticket holder — of C.E.Europa (Els Escapulats) football/soccer team. Costs only 50€ for a season and I get to watch every home game for free; and away games cost only 5€. Even currently, with covid restrictions applied, there are matches open to the public. There are 15 (fifteen) local football/soccer clubs registered in and around Salamanca. Sports fans, of all nationalities, can't help but jabber on about their team's perceived strengths and weaknesses, and the fallibilities of the game officials! He'll pick up some great, useful phrases and likely have a blast.
However, I am still at a loss when it comes to practicing català on a regular basis — even though I have several Catalan friends; covid restrictions put an effective stop on intercambios — so, we swap messages on WhatsApp. Everyone in Spain uses WhatsApp. If he happens to meet a local he knows he can trust then they could swap numbers — he writes messages in Spanish, they return in English.
However, has to be said, non-native languages are best learned in the cradle (too late for that strategy) or in bed with a lover. Dating might be a way forward. I mention this because that was precisely my partner, (of 17 years) experience. Born in Washington D.C., raised in Mass. and schooled in Vermont, like your son she attended Salamanca, and, during an excursion to Barcelona, met a guy who spoke next to no English, and fell in love. She returned to the States for a year, finished her degree and enrolled for her Masters in Barcelona. (For her, the challenge was learning català. Which she did — so well so that some neighbors will not believe she is American.) They married and produced a son (with joint U.S. - Spanish citizenship) — who speaks Spanish, català, English, French, Dutch, and a good bit of Polish.
Hope your son finds a practical solution/workaround.