Not sure I understand Bobo's tangent onto all UK school children have poor work ethic, so I'll stick to addressing the Op's points (if OP ever returns!)
The increase of workload is hard, but work with your DS to strike a balance between that and your personal downtime needs.
If he enjoys his extra curricular activities, try and encourage him to maintain them, albeit know when to bail on a session if things get too heavy. Doing 'other stuff' helps with the academic side for lots of children, and can improve their multi task skills too to complete work more effectively
Do plan things for some weekends. But, again, if things get too heavy, be flexible. For example, a child that has worked 6 hours on study on Saturday, surely has some flex on a Sunday to enjoy/go out with the family? If this cannot be the case in any weekend out of 4, then there is an issue and the school should be challenged with volume of work, or his ability to manage. And there will be peaks and troughs throughout the school year - no one should have to just look forward to the end of terms when exams are done and teachers are on happy time.
Work with the school on support strategies, until routine and ability to manage gets easier. If he's driven, he'll want this too.
Weekdays - sounds about right, many families won't see their children from return to bed apart from feeding them, especially if they have added things to their school day.
Finally, if you have more than one child, think of the impact on them, to adjust your family lifestyle so much around DC1 at weekends, that subsequent DC suffer. And, you will need to forfeit weekends for them as well when time comes? This then becomes the norm for them that school is all about study, no room to step out the bedroom desk?
This is not what education should be about - that 7 years are sacrificed for head down during school terms to come out with circa 10 good GCSEs. But compromises do need to be made at peak times of pressure. This is not a schools issue alone, but a misinterpretation by some parents that number of hours worked = happy, balanced child who has good HE and subsequent job prospects. Never mind he/she's a ticking time bomb of unhappiness due to such an unfulfilling school life.
I am not disputing that the UK system has its challenges like many other countries, but don't think the OP's question was about whether the french system is more superior. I also don't think it's helpful to state 'commuting is rubbish'. Course it is (and for some, truly horrific and perhaps needs to be changed), but for most of our children, they get on with that aspect of life too. And learn to use it to their benefit where wifi allows!