Thanks for your question and welcome to ADHDCentral.com.
I think there are many parents out there who can relate to your situation. When children with ADHD become adults, there is often a lot of conflict. Often, they are still immature, some resent taking medication (as your son does) and they want to be independent and make their own decisions, even when those decisions have disasterous results.
Letting go, however, doesn't mean you have to let your son live on the streets. But it is important to make him, not you and your family, responsible for his decisions. Dr. Ari Tuckman, in an interview on teens and young adults with ADHD, explained it this way, "All decisions have consequences. Some decisions will create a harder situation than anticipated and teens with ADHD must learn through the pain of poor decisions. While it is hard for a parent to sit idly by and watch their child learn through pain, it is exactly this type of experience that will help them learn the lessons needed to navigate adulthood. Telling them about the pain they may experience is not enough; some teens and young adults with ADHD must feel the pain and learn life lessons the hard way. It is important for parents to let go enough to allow their children to experience the pain."
By taking on the financial responsibility of your son's decisions, you are enabling him to continue making poor decisions. As difficult as it is, it is probably time to let go, at least in part. Dr. Tuckman suggests, "Parents can provide necessities, such as food and shelter, but cut off additional funds for non-necessities like cell phone service or money to go out with friends. These extras may be motivators for the young adult. He or she may want cell phone service enough to get or keep the part time or full time job to pay the monthly bill."
Start small if you need to. Begin by telling your son what your expectations, such as holding and keeping a job, and what will happen if he does not meet these expectations. You may not be ready to tell him to leave the house but you can stop paying for his decisions.
I know this is difficult. I know he is your son and you love him deeply. But in order for him to grow and learn responsibility, you will need to stop taking on the responsibility when he fails.
For more information:
Parenting Teens and Young Adults
Adult Children with ADD Living at Home
I hope this helps. Please let me know how you are doing.