It looks like the only way to motivate your son is to givehim a deadline when you expect him to move out. Put it in writing and have him sign it. I would also expect him to domany chores around the house, help you with cooking, cleaning and anything elsehe needs to do to contribute to the household. If you make your house too welcoming and you do not give him any workaround the house he goes back to feeling like a child. He is thinking mom cantake care of this. To encourage him tobe independent you have to stop doing things for him such as laundry, cooking,cleaning after him. Tell him that youbelieve in him and that you know that he will find a job. Do not change themoving out date no matter what happens. If he has no job before the move indate he can move in with a friend. Youshould have a meeting with him both you and your husband and tell him exactlywhat you expect from him and give him a timeframe. Get it all down in writing and ask him tosign the move out notice. Do not remindhim to get a job or push him because it is not your job. It is his job to finda job and move out of your house.
I think that you are right and he does sound like he has ADHD. Adults with ADHD mature slower than ones who do not have the disorder.
His doctor can also diagnosed ADHD and treat it with medication. Medication for ADHD really works. If he does not believe in therapy he can still see a doctor and get on medication. I would buy him the book "ADHD and loving it" or the one "You mean I am not lazy stupid or crazy?"