This is probably too late to help, 3 months after you asked your question. I just joined the forum today. I teach at HSMSE.
The HW load at HSMSE is considerable, I'd say a minimum of an hour a night, probably closer to two hours, though there will be light and heavy days.
We have had numerous ADD students in prior years. There are programs in place to help students facing this challenge.
Starting in Fall 2012 we will have a class for all Freshmen on how to adjust to high school, manage time, assignments, and so on. The teachers will act as additional guidance advisors as well.
I'm not familiar with iSchool, but HSMSE does offer a challenging curriculum and our seniors are well prepared for college when they are done, having had an exposure to high level science, math, history, english, engineering instruction.
A student must work hard to succeed; there is no way to avoid that, but we try to keep the pressure low.
Good luck to your son and yourself as he moves up and on to high school.