@JohnnyUI can totally relate to what you are going through. We have basically went through the same thing with our now soon to be 24 year old son (my stepson). Was also diagnosed with ADD is high school but refused to take the medicine. Had decent grades, but could have had much better. He also struggles with authority because he thinks he knows everything about everything and is very argumentative. He goes through a cycle about every 4-6 months and it is like a roller coaster ride. The last time he moved out (while we weren't home) , we told him it would be the last time we let him live here because we wanted him to stay with us to help him get back on his feet by getting all of his fines and court costs (has been in trouble with the law) paid along with his other bills paid. We keep hoping the switch will flip some day.
It is very hard and painful to put your foot down with your children/grandchildren but sometimes it has to be done. I am thinking of your family and hopefully everything works out for you