Opinions on recommendation letters

My son’s first choice teacher for a LOR is his French teacher. He really likes her, did well in her class, and had her for two years so she probably does know him best. Great. However, he’s applying as an applied math/statistics major so I suggested that he also ask a STEM teacher and he asked his physics teacher (while he’s a great math student he didn’t feel he has that strong of a relationship with his jr year teacher). So, this is good for schools that allow two LOR. But, some schools on his list will only look at one LOR. In that case, any opinions on which he should use?

For schools with only one LOR, choose the teacher that knows him the best as a student, which sounds like his french teacher.