English I think really depends on the teacher. I had a really easy senior year English class. Really, we did basically nothing the second half of the year. I think I had a 99.9 in it. I still got a four on the AP in spite of doing very little. That being said, your son could probably take and do well on the AP English exams with little more than a review book. They are based more upon what you've learned all four years of high school, not just your senior year. The honors English classes (at my school, it was standard, honors, and GT, Gifted and Talented, for English the first two years, then GT dropped off and it was AP. AP and GT classes were worth the same amount as a weighted GPA) were often just as much, if not more, work than AP. Especially senior year.
I would be pretty worried about AP stat. I mean, he's obviously good at math, but when I was in high school, everyone complained about stat. They made that exam even worse a few years ago. And I took it in college...and complained the entire way through. Those who knew me and were on this thread remember.