It sounds like he’s not getting the level of support at school that he needs. In middle and high school especially, educators think students should be “responsible” and “accountable” and “motivated to do well.” They have narrow definitions of those terms that convince them that kids like ours are just lazy and defiant. Every kid wants to do well. If they’re not able to achieve it, then there’s something more going on.
I imagine his overly-optimistic outlook is some denial and self-preservation. Because, again, he wants to do better than he’s doing. Skipping class may also be an attempt to not fail in a twisted way (he can blame not being in class instead of feeling like there’s something wrong with him or he’s dumb).
I don’t have advice here really, my son just barely scraped through his freshman year because the school doesn’t support him enough. And they know I write books on ADHD and am a very educated parent on learning challenges and they still believe he’s lazy and irresponsible. Even his special ed teacher. They just don’t understand executive functioning and the depth and breadth of the impact of those deficits.
I think every state in the US has online virtual public school for free now. That may be an option, if you think he’d do better in a different environment. Maybe it takes him an extra year — some kids just aren’t good at school, no matter how smart they are. And, leaving it up to him to go to tutoring probably isn’t going to result in a good outcome. He needs to be reminded at the time he needs to go and he needs to know it’s not optional.
See if any of the accommodations in this list sound like they would help, and then propose them to the school:
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism