You are immersed in a codependent situation. You can google codependency for lots of information and ideas about dealing with this common problem. Here's on good site: http://www.nmha.org/go/codependency
Codependents try to take care of a person who is experiencing difficulty, but the caretaking becomes compulsive and defeating. Your repeated financial rescues allow your son to continue on a destructive course.
Characteristics from the site's checklist:
An exaggerated sense of responsibility for the actions of others;
A tendency to confuse love and pity, with the tendency to “love” people they can pity and rescue;
A tendency to do more than their share;
A sense of guilt when asserting themselves.
An Al-Anon group near you can give practical support in recovering from co-dependent situations, and I know a number of people who have gotten good, concrete results.
In the meantime, you might adopt a stock phrase to tell your son, like, "I hear that you want money again. And no, dear."
DO NOT make excuses or give reasons for saying no. This will only weaken your position, and convince your son that he can argue or manipulate himself past your objections. If he gets angry, tell him, "I hear that you are angry. And my answer is no."
Good luck. You are understandably worried for your son; unfortunately, your protecting him from the consequences of his own choices will only put him at greater risk over time.