S.M.
Sounds to me like he is depressed --- it tends to manifest itself a little differently in teenage boys. But it also seems like you've done about all you can, and at this point, you need to push him out of the nest. You can't baby him forever, even if he acts like a baby.
In my opinion, the best thing you can do for him is nothing. If he buys his shoes too big, let him wear them too big. If he breaks his glasses, let him tape them together, and if he loses them, let him go without. Stop trying to get him to study for the GED, stop trying to help him do anything. Let him get away with not doing his chores. But the moment he asks for busfare, or for money to pay his cell phone bill, or for new videos or computer games, or if he asks you to buy particular snacks that he likes, remind him that you have told him if he doesn't do his share of the work, he doesn't get any consideration, and you are not buying him anything.
When I stopped trying to get my then-17 year old to do his homework, and stopped trying to get him to get decent grades, and stopped trying to get him to do his chores properly, and stopped warning him about all the problems he would face as a young adult if he couldn't learn to be responsible for himself, he finally finished high school during the summer after all his friends graduated. He is still not good at doing his chores, or getting places on time, or being responsible, but about three months after I started charging him $400 a month for room and board, he moved to Southern Cal to live with a couple of his friends, and has started attending community college. He is trying to support himself as a tattoo artist, although I think he realizes that I am right--- he won't be able to pay his rent consistently on that kind of "underground" income. Instead of telling him over and over again that it won't work, I am just kicked back watching the developments. I think he will find his way, and, like your son, he's a good kid, and I'm proud of him.
(Oh, and, make sure your younger kids see the problems he brings on himself by failing to do what he needs to do.) Best of luck.