But the OP’s son is interested in international business, where showing language proficiency would be important.
@Techno13, Are there any summer language classes available in your area (at a community college, if not through his high school?) Would be nice if he could do Latin 3 (or equivalent) this summer, AP Latin next year, and then get started with Mandarin senior year (and that way get the great recommendation from the teacher). Or, get started with Mandarin this summer and continue through Mandarin 3 in senior year?
My D will not major in STEMor business but I urged her to take physics just to cover her bases. Her high school doesn’t offer honors physics so she had to take AP physics 1. She managed to make an A even though she isn’t very mathy and didn’t particularly like it. AP physics 1 is algebra-based. Since your son is already taking AP calculus BC as a sophomore, he shouldn’t have any trouble doing well in AP physics 1, even if it’s not his favorite subject!
Another thought…if your son can’t do summer school in foreign language maybe he could take a summer physics class (honors or AP 1) and then have room in his schedule for AP bio and AP chem? I think having a rigorous STEM-heavy record can compensate somewhat for a lighter schedule in FL (or vice-versa) but if he’s applying to competitive programs he probably shouldn’t show deficits in both areas (I.e., no physics and 2+2 or 3+1 FL).