That sounds like a tough case. Sounds like he’s skeptical of everything, which could certainly be part of anxiety.
The way we explained medication to our son was to let him know it would help him with struggles in school and at home (listed specific things he had complained about or was upset about). Hearing that it could help him in areas that upset him was enough for him to buy in (he was only 6).
THere’s some great tips and strategies on how to handle teens refusing treatment in this article, which you can certainly implement with a preteen as well.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism