Part of my job is writing policies based on the latest COVID-19 information, and I haven’t heard anything about ADHD vs. COVID-19 mortality. There’s no physiological reason for there to be a connection. The most significant factor is age.
My best advice for your son is to try different types of masks. But remember, the best preventive measure is to limit contact with other people. Keep up all of the various precautions; they work together, not separately.