Depending on the school and major 3-4 years of a foreign language can meet college graduation requirements. I know 3 (HS years, college semesters) of one for a BS and 4 of one or 3 plus 2 of two languages for a BA at one school. It can be useful, again depending on major and goals, to finish the language reqs in HS. Several variables here- is continuing in Mandarin worth it? You/he would need to research possible colleges for their requirements.
Yes, my S has completed 3 years of Mandarin
Check on the usefulness of that 4th year for his getting into, meeting college graduation requirements at schools on the radar.
depending on the state that you are in, if there is a particular good state school that you are targeting and the current GPA won’t get you in, perhaps you can get your kid to start taking CC classes as a senior (or even before). If you plan it right, he or she could apply to that hard-to-get state school the following year instead of waiting 2 years.