F.C.
Unfortunately, he does need to be allowed to make some mistakes so that he can learn but....not ones that are going to change/wreck his life forever. Start out with small things and build trust. Have him run small errands for you and maybe have him set up a bank account (with you as the co-signer) and have him earn money at home by doing chores. If he makes mistakes, they will at least be small ones and it will give you an opportunity to teach him better ways of living. As he makes good choices, you will learn to trust him more and it will be easier to let go because you will know he has at least some basic skills in life. It also might not be a bad idea to get him some counseling or hooked up with a life coach who might better explain to him why he needs to be patient with you (and himself), as well as how to live with a learning disability as an adult.
Hope this helps!!