I volunteered as a Spanish interpreter, got certified, and now working as a full-time so I could tell you a bit about interpreting. Spanish interpreting seems to be a more unusual experience compared to and involves different responsibilities/tasks than scribing (at least as far as I know). Since your son volunteers at a clinic, he will get exposure and insight into the patient's side of healthcare and get the opportunity to advocate for the patients and their families.
That being said, something you could talk about with your son is recognizing one's own limitations. You mention that your son is not a native speaker, which could potentially do more harm than good depending on his actual level. Another aspect to consider is the difference between interpreter and a native-speaker, the latter one could still be helpful in a lot of scenarios, but there are differences in terms of interpreting for meaning, tone, emotions in an encounter that a native-speaker could potentially miss. If the clinic your son volunteers at is aware and ok with your son's Spanish level (i.e. having some rather than none), I don't see a major issue, but if his Spanish is not there you might want to speak to your son.
If you have any specific questions PM me