Good morning sefrif, welcome. I'm glad you found us, there is much comfort here. I think your situation is a fairly common one in our little corner of the world. Some kids have a real failure to launch at around the age your boy is. I think you have received good advice. Sometimes as parents we have to keep readjusting to our kids behaviors because they don't follow the usual trajectory, so it is just as much a learning experience for us at it is for them.
My input would be exactly as you just stated, make an evaluation by a professional a requirement for living with you at this point. It seems that many people have more then one diagnosis following an ADHD diagnosis, there is a great likelihood that he needs some kind of help. His refusal to go is irrelevant now, these are your boundaries and rules because it is your home and your life, he is an adult and you hold all the power. You and you husband should sit down and figure out exactly what it is you want and what it is you don't want. Find an amount of time that feels right to you and make that the date he has to meet all your requirements.
If he has no social skills, going out into the world would be scary and he might be acting out so he can avoid the inevitable. If it were me, I would be firm with my boundaries but remain compassionate about his real or perceived fears. I think a counselor or therapist would be able to help not only him, but you and your husband as well. So you can all come to terms with exactly what his mental and emotional limitations are and how you can help him and what you can expect from him without enabling him. Finding that line between helping and enabling is tricky and that's where a professional can help you.
I believe boundaries set by you and your husband are the first step. Getting evaluated, going to family and individual therapy, setting a date for him to have a job, be in school or both, requiring him to make contributions around the house, to be a part of the family by being a contributing member, being respectful of you and your husband, all of the requirements that you feel are necessary for ANYONE to live in your household. I think having this conversation you are about to embark on is timely and will ultimately be good for him too, boundaries make everyone feel more safe. Asking him what he wants is important too, what future does he envision for himself? Talking about his fears and concerns, although would be great if he were willing to do that, is likely not going to happen, given his gender and his age, which is why I believe a therapist would really benefit him now. He's still young and if he gets evaluated AND he gets therapy, he will be able to start his adulthood on firm ground.
I know this is tough on you and your husband, but you have good insight into the situation and you have the willingness to shift and adapt and be firm, so you are in good shape. I wish you good luck in your talk today, hopefully you will come to some agreements and be able to move on with a good plan. Keep posting, it's helpful. I'm glad you're here.