I don’t know of any boarding schools that include courses taken elsewhere in 8th grade in the GPA, and few include grades from other schools period. Policies on GPA and transcripts will typically be laid out in the student handbook (almost always available online) and/or the school profile put together for college admissions officers (also often available online, usually in the college advising section of a website).
So I wouldn’t sweat a B. But schools can and will rescind admissions offers for grades that seriously tank in the last semester, so he shouldn’t go off the cliff completely. It’s probably a good time to remind him that he’s going to have to be self-motivated and responsible for keeping himself on track at boarding school, even when the assignments are boring or he’s got more exciting things going on.