Suggestions from years of 'mixed' experiences
On a practical note re organisation -
Get an academic year wall planner and put on it the important dates eg. UCAS deadlines ( including ones the school sets for PS checking etc), mock exams, uni visit days, summer exams etc etc.
If you don't already try to have a fixed time each week to chill out with him - either one of you and him or as a family etc doing something he enjoys. It helps keeps relationships going even if things are tricky and gives him a time to talk if he wants to. Doesn't have to be for hours! but it also demonstrates to everyone that it's a priority.
Get him to schedule his time now so he has agreed amounts for socialising, computer games or whatever he enjoys so it's not a constant battle.
Agree a method for him keeping you informed about academic progress so he knows you are checking but again it's not a constant issue.
Book some treats to look forward to after probable stressful times/ to give a break - maybe some family ones and some with his friends. And put them on the planner too
Try to welcome his friends for socialising at your house ( can be trying at times!) It's a good way of getting a feel of how things are going.
If he doesn't already get him doing a bit of his own washing and cleaning ( especially the bathroom) and basic cooking ( can be done as a chill out time activity too) ready for flying the nest. Also reminds them that if they want independence they also have to take on responsibility
It is an important year but whatever the outcome it's fixable so try to enjoy it as well amongst the stresses.