Honestly, I would say Stop trying to get him to write assignments down. I was recently in school, And I can say, even if I had been able to write things down they wouldn’t have gotten done anyway. I have only just been diagnosed with ADHD and it makes my life make so much more sense. Especially the issues I had in school. I agree with the person above, I found taking a picture of the board to be the most helpful. remembering to use your phone may be easier for him than having to remember to write something down.
Maybe it I’d had medicine school would have been easier. But trying to force someone who is Neurodyvergent to act like someone who is Neurotypical isn’t the way to go. All that’s going to do is make him feel more stupid, and ingrain ideas in his head of who he is, because he can’t do X,Y, Z. i would try to find ways around it, See if he’ll be able to take a picture. Maybe a friend can email him the assignments. Or the teacher can hand him a written(typed)copy of the assignments for the week. It’s more about finding way’s around it that he can handle, than trying to force him to do things he just can’t.