I'm experiencing similar with my dc & from talking to teachers and parents I don't think this is unusual. A few reasons that have been suggested to me:
High achieving students can often get through GCSEs with minimal effort. Having not got into good study habits or learned to work hard, they sometimes crash at some point in 6th form when minimal effort proves insufficient.
Online/screen time can easily get out of control, and 2+ hours a day online is like having a part time job to fit in on top of studying and other commitments.
Depending on the school, 6th forms often give students a lot more control and freedom - free periods, less contact between teachers and parents, longer deadlines for homework. Some thrive on this, some use it as a reason for not working.
Some students do the uni visits, get offers and then are so focused on what they're going to be doing next autumn that they take their foot of the gas in terms of what they have to do now in order to get there.
It's common for stressed or anxious students to bury their heads in the sand about A levels and ignore the work building up.
I'm sure there are others, but if you can work out if any of these are issues for him - maybe talk to teachers if possible as well as your ds -then you might have some ideas for how to help. Could you limit screen time/take in screens at a regular time at night/put a timer on your router/have screen free days? Find ways to help him cope with stress? Help him with organisation/revision timetables/a place to work where you can see him, etc? Keep in regular touch with teachers if there are any issues at school with coursework, attendance or homework?
Motivating students to work harder without coercion is a much tougher job - would love to hear if anyone has managed this!