I don't know what ADHD is. Would you explain?
The Gestalt Centre is putting on a course in June, facilitated by Frances Verrinder, entitled working with adolescents and their families. However you may not be in UK and this is probably late for you.
To work with anyone in a Gestalt way is to look for what is figurative for them at that time, and respond accordingly. Adolescence is, as I am sure you are aware, a most confusing, self-conscious and frightening time. Adolescents are struggling with a state inbetween that of child and adult. They swing from one to the other. Their changing bodies cause great anxiety and self-consciousness, and they are often so testing that compassion is very hard to maintain, especially for parents. Good empathy is particularly important. A lot of this is common sense.
I would be interested to know more about your work, and what you do do with your client, how Gestalt fits in. I look forward to more from you on this noticeboard.