Agree with him
how much time is going to be his to do as he pleases eg 1 morning a week when he can get up as late as he likes, hours on ps4;
how much on constructive chill out eg hobbies, music , sports, walking, gardening, reading, model making, cooking
How much on 'academic'-specific A level prep or a project, reading around the subject ready for Uni applications
How much on employment - paid or voluntary ( which will also start building up a CV for future employment).
Hopefully! after the initial negotiating you won't have to keep nagging every day.
Body clocks do adjust so keeping to within a couple of hours of school getting up time and then getting back to routine a couple of weeks before school has been our aim.
Exercise, fresh air and healthy eating are old fashioned goals but still a useful prep for the stresses of A levels IMO