My husband's family doesn't acknowledge his attention deficit, and they don't take him seriously. How should we handle this?
I'm glad that you said "we" and understand that the two of you are in this together. His family should be educated about the ins and outs of ADHD, to understand how it affects your husband's behavior. They should know that he can't control his distractibility, poor planning, or inability to remember things.
If your husband's family believes that ADHD is not real, a talk with your husband's doctor might help. Once the family "gets" ADHD, he should tell them how hard ADHD is to live with, and that it bothers him that they don't take him seriously.