My answer, which echoes some of the best advice from the other posters here, is to let him face reality. Let him fail. Give him an ultimatum: shape up or ship out.

I joined the Air Force after I graduated high school. I was a satellite operator which was some of the most rewarding work I have ever done. After I separated I had a hard time finding a job (5 years ago or so). Some friends supported me for a while, but I grew lazy, and did not do much. So, I found myself homeless when I was 26. I got myself together, and got myself a good job working on computers. Now, I have a rewarding job working as a Unix System Administrator (Linux has been a hobby of mine since I was 16). Your son's path may be different than mine, but the important thing is to get him started down it.

Calling your son a loser is not necessary. Nor is letting him stay in his current rut. It is a false dichotomy. Love, however, is necessary. Not all love is soft, some of it is tough. You want your son to succeed. In order to do that, you need give him the chance to fail. He is an adult now...treat him like one.

This is not pop psychology, but what wise men have known for centuries. From the Discourses of Epictetus:

"To those who fall off from their purpose

Consider as to the things which you proposed to yourself at first, which you have secured and which you have not; and how you are pleased when you recall to memory the one and are pained about the other; and if it is possible, recover the things wherein you failed. For we must not shrink when we are engaged in the greatest combat, but we must even take blows. For the combat before us is not in wrestling and the Pancration, in which both the successful and the unsuccessful may have the greatest merit, or may have little, and in truth may be very fortunate or very unfortunate; but the combat is for good fortune and happiness themselves. Well then, even if we have renounced the contest in this matter, no man hinders us from renewing the combat again, and we are not compelled to wait for another four years that the games at Olympia may come again; but as soon as you have recovered and restored yourself, and employ the same zeal, you may renew the combat again; and if again you renounce it, you may again renew it; and if you once gain the victory, you are like him who has never renounced the combat. Only do not, through a habit of doing the same thing, begin to do it with pleasure, and then like a bad athlete go about after being conquered in all the circuit of the games like quails who have run away.

"The sight of a beautiful young girl overpowers me. Well, have I not been overpowered before? An inclination arises in me to find fault with a person; for have I not found fault with him before?" You speak to us as if you had come off free from harm, just as if a man should say to his physician who forbids him to bathe, "Have I not bathed before?" If, then, the physician can say to him, "Well, and what, then, happened to you after the bath? Had you not a fever, had you not a headache?" And when you found fault with a person lately, did you not do the act of a malignant person, of a trifling babbler; did you not cherish this habit in you by adding to it the corresponding acts? And when you were overpowered by the young girl, did you come off unharmed? Why, then, do you talk of what you did before? You ought, I think, remembering what you did, as slaves remember the blows which they have received, to abstain from the same faults. But the one case is not like the other; for in the case of slaves the pain causes the remembrance: but in the case of your faults, what is the pain, what is the punishment; for when have you been accustomed to fly from evil acts? Sufferings, then, of the trying character are useful to us, whether we choose or not."

Finally, a scene that was personally meaningful to me while homeless: