I'm not sure where you got your ideas about JHGS being more relaxed on the boys I guess I don't have experience of any other grammar school to compare it to, but from my experience of the school soo far, your DS will not have any chances to relax. The school has high expectations from the boys and they're expected to do lots of homework (2 hours a night) and have very good organisational skills.
Gosh, the poor things! I don't live in Bucks but have children at superselectives in Gloucestershire and the workload is nothing like this..my year 7 barely has any homework, and my year 9 nothing like 2 hours a night. My sixth form daughter maybe does just about that, and did so in the run up to GCSEs but certainly not before. A friend with a son at a GS in Bucks (not one of these) reports similarly small amounts.
I wonder why they are putting such pressure on in year 7- the curriculum doesn't demand it.
You could always ask of have a look at their homework policy- they have to have one.