Is this really a defiance issue? Generally, the answer is most often no. Determine the reason behind the behavior (refusal) and then you can improve the behavior by addressing the reason.
This could be slow processing or auditory processing issues, executive functioning deficits, working memory challenges, or his brain getting ahead of his body.
6 Truths About Child Behavior Problems That Unlock Better Behavior
Penny
ADDitude Community Moderator, Parenting ADHD Coach & Author, Mom to teen w/ ADHD, LDs, and autism