I wouldn't write off ADHD.
It can manifest in "poor behaviour". School can be really disengaging, it's boring and tedious and monotonous. You can't make your brain stay on task. It can also make you impulsive, ie. I am bored here, I will skip this class, instead of thinking: If I skip this class now I will be in trouble later. You don't have the same perception of time. You live in the now. Whatever comes later is a future you problem.
Also being unable to do homework, again, boring, tedious, removes autonomy. Traditional punishment measures don't work, like removing phones etc. because the brain is searching for dopamine so it can work. If you're removing a source of dopamine, then it's basically putting you into standby mode, and you can't start or complete tasks.
Equally, if you're overstimulated, it can be hard to break away, and start to make new routines. Being told to do your homework won't make you suddenly able to do your homework, it really needs the right guidance to make it fun, engaging, and to help stay on task.
If he isn't getting the novelty, excitement, or urgency factor out of a situation, then he won't do it.
If these needs have been missed, and nobody can tell you why you're like this, you can easily be written off as naughty, ill-equipped, poor work ethic, and you just end up with anxiety, and that leads to avoidance behaviours, like skipping lessons.
Obviously nobody here can diagnose your son, but I just don't think it should be written off as a possibility when the primary goal is to get him to stay in lessons and engage and not be so mentally checked out before his life has even begun.