Ok, so having an 11 year old who can't do normal activities of daily living, without a tantrum means that something has to change, if possible.
Will a label help? Hard to know...
What has been tried for the ADHD? Medication? Parenting approaches? Friendship club? Organizational skills training at home or in club?
What is the rest of the house like? How much are you willing/able to do for him? DS and I had an agreement that his backpack was not his 'personal space' - that meant I went through it every day and made sure that no PBJ science experiements could grow, or that no school permission slips or homework sheet went unnoticed. No everyone's style, but I felt that the best way to teach him was to give him a taste of the benifits and slowlllllllllllllly wean him into self management.
Love and More Love,
Grinity
