Vik tbh I think the best thing you can do is sit down with mum & talk about all of this.
ADHD is hard enough to deal with as a group let alone as a minder with other mindees to think about!
His age has a large bearing on how to tackle problems & mum may have strategies already in place (at home or with school) that could give you more idea & help.
I'm not always sure reward charts work that well .. as at the end of the day the child will switch off when they realise it's the only thing being used to try & 'modify (for want of a better word
) their behaviour.
What truly interests him & gets his attention ?
Please ... let us know how things go & don't forget we're here to help let off steam
