Your sons needs might not qualify him for an ehcp. They're notoriously hard to get. But, his one page profile should have agreed accommodations on it.
Does the school know the days he doesn't take meds? They may think it's a choice he's making without realising his thought processes will likely be less organised, more impulsive and harder to manage on non meds days.
One of the things your son does need to learn is actions have consequences. Otherwise he is being set up to believe he can use his adhd as an excuse for poor behaviour for life.
Action - not taking medication
Consequence - handwriting messy, pen use revoked.
While I don't agree with pen licences and my adhders both never got theirs, it's a very easy example to use as your son has experienced it.
I have one on meds and one waiting. On the days where Ds complies with the agreed medication regime, I always defend him if he gets unfairly sanctioned. Eg rules are not made explicit, he gets overwhelmed and ends up making a poor choice. Because he is showing he is trying.
On days he does not, he takes the consequences of poor behaviour because he has opted not to help himself first.
If the pen licence is a hill you want to die on, have it written into his support profile. Both of mine have no sanctions for lack of homework completed because by the time they get home, they need to regulate themselves and have a strong resistance as school work is for school in their eyes. They're encouraged to attend homework club but this is not a compulsory activity so cannot be forced by me or school. This is written into both profiles and I've explained the consequences to our family if we try to force them to.
It might be worth sitting down with your son and asking why he doesn't like taking the medication. If it's about him feeling different, you're right to reinforce that no one knows as he's taking them at home. But also look at famous people who have adhd - Simone biles, Justin timberlake, Michael Phelps - a quick google search through up quite a few. And if he's not engaged in extra curricular, I recommend. Mine do competitive trampolining and music lessons. It's made such a difference.