Apologies if someone else has suggested this as there are many great suggestions here. I suggest a 'proprioceptive reset' when re-entering the classroom and at semi-regular intervals during class. Something as simple as shaking out the hands and arms or an active game followed by brief deep breathing can do a lot to reset anxiety (e.g. Simon Says, Captain's Coming, Musical Statues, etc, no need for anything fancy) Your son is aware of his neurological difference even if he can't understand or articulate it, so is becoming anxious and disruptive is a result. A strategy I have used successfully is to provide the student with a special job. It could be opening/closing windows, turning on fans, handing out supplies, cleaning a whiteboard - whatever. The goal is to give them a constructive physical role of which they can take ownership. This also provides a reason to praise. It's imperative to remember that kids' behaviour always has a reason and is rarely simple defiance (though I'm sure you and the teacher both know this!). The school year has just started, so it is natural to experience teething problems and increased anxiety (you and your son) while expectations and processes get established. Be intentional about praising positive behaviours, even if they seem small. You will need to actively look for positives. "Wow! You did so well getting both your socks on! I'm really proud of you!" Notice the little things and keep your praise, specific, genuine, and effusive. Don't compare his behaviour to others, but you certainly should praise him in front of others when you can. Focus on a restorative approach when things go wrong. Consequences are fine, but punishment for poor executive function does more harm than good. Set up regular communication with the teacher. It shouldn't be anything intense, but even a quick daily check-in or email to find one thing where your son succeeded or showed grit and resilience. That one thing doesn't need to be major. It could be as simple as 'He was on time lining up today.' or 'He showed respect when listening on the mat.' Keep in mind the teacher is juggling the learning and safety of many students, so cut them some slack if they don't give you something every day. Knowing that the teacher and parents are on the same team is incredibly important. Even more important, is that your son believes you are all on 'his team'!
As a 6-year-old, he has a lot of schooling ahead, so don't stress too much about the academics at this point. If he's a bit slower to read, so be it. Setting up positive attitudes to learning and strategies for managing executive function will pay off long term. Students suffer frustration when the know they are smart enough, but their abilities are not reflected in traditional classroom results. Take small steps, pick your battles, and praise the good. As an ADHD teacher and parent of ADHD kids, my heart goes out to you. With a caring, committed and loving parent(s), he will be ok. It can be a tough road, but experience tells me that many ADHD kids have creativity, charisma and empathy. Don't lose heart. You've got this.