Help for my 16yr old Autistic and ADHD son to be able to access learning during lessons

My son is doing his A'Levels, having got through GCSE's with no planning, revision and by not knowing much of the content (as he actively disengaged in lessons).  He is now reaching a critical point as he can't continue in this way and still succeed at A levels.  I get contact from the school most days as he hasn't done his homework, has his head on the table in class and isn't performing in topic tests.  He is oppositional to discuss how we can support him to plan his homework and get a structure/schedule in place for bite sized learning.  His teachers want to help.  He is extremely clever but can't seem to apply this.