Unfortunately ADD contributes greatly to a loss or impeding of executive functioning in a lot of cases. I am sure your husband knows that he must work toward being re assessed...but his procrastination and inability to put thoughts into actions stops him from moving forward.

Maybe try to get a friend involved who can take him to the medical appointment or compel him to keep the appointment so as to get started on the medical help he desperately needs. If he is left to his own devices he may just continue being caught in the perpetual procrastination/motivation trap.