@RightCoaster , he is going to attend more extra help sessions. He has met with the teacher several times. (I was at one meeting too.)
@lookingforward , good idea re summer class or cc.
Then, it sound like TOK would be a great class for him to take. Actually I think it’s a one-year class. He could follow that with Philosophy SL. Both tend to be impressive in that they require a lot of critical thinking and rigor. And they surely would help him with writing - it may not necessarily be long, but the architecture’s frame must be steel. So it may actually work well for him.
How is he handling his current schedule, from a stress standpoint?
@MYOS1634 he is good. He handles his work fine. He is busy, but he is not an overachiever. Underachiever is more accurate, with a good helping of kick up the backside from us parents, haha!. He plays a couple of sports, has summer jobs, one after school club at our insistence. He is very social. He wants to be a peer leader next year, which is a mentoring program for freshmen. He is a solid high B-low A student, apart from this stats class. He has zero interest in prestige, but of course we want him to go to a good college. He has no test scores of any kind yet, so I haven’t made a big effort to get a real list going yet.
Update: My son has been going to extra help twice a week, and I hired a tutor. His teacher says she is happy with the improvements she is seeing. I will post an update at the end of March, which is also the end of the quarter.
My son got a 4 on AP Stats!!! I am so proud of him, he studied like a demon. It’s a shame it took him so long to get the hang of it, because he unfortunately averaged a C, except for the last quarter. At least he will probably get credit or placement for it, as he is thinking he might major in economics. Anyway, his hard work paid off. 
Congratulations to him!