I would think drugs. Drugs really mess up your sleep patterns. Now if he'd never taken drugs before, I'd tell you to take him for a check-up. Since he did, my best mom's intuition guess, which COULD be wrong, is that he is using again. Jobs are big place to find drugs, but so is just walking out the door. It is a choice your son is making. The onus is on him. Sleeping 14 hours is so not normal. I am thinking he started using drugs before you knew it, unless he has a serious illness or if he has even done this as a young child and was diagnosed by a neurologist with narcolepsy. Sadly, by experience, I am guessing he took drugs way before you even thought of it and you just didn't know he was sleeping for that reason. Our drug using kids are very clever at hiding t he fact.
When my daughter used drugs, and we were in deep denial and thought she was only smoking pot (what a joke that turned out to be...a bad joke), she was up all night (although we didn't know it) just so that she would be high while we were asleep. Then she could not wake up during the day and slept constantly. She had moved on to meth and psychedelics and, to our shock, we found out after she quit and was willing to talk about it that she had even tried heroin. So we as parents can be fooled. We WANT to not know!
You did all you could. The fact is that he has to work and function in society or he will be crippled even if he IS drug free while you coddle and nurture him. He's too old for that and none of us can live forever. He needs to learn to take care of his own drug problems. I would cut off the money completely, the cell phone, the face book...make it hard for him to use, if, in fact, you believe he is using. To find out though, you need to check his cell phone messages, his Facebook, go through his entire room and check under his mattress and in cracks and crannies (even in his shoes), and see if you can find signs. You are not invading his privacy. In fact, because he broke the law and lives in your house under your rules, he has no privacy. You are trying to find out what is wrong so that you can save his life, or try to.
The actual saving will be totally up to him. Nobody can stop his own self-destruction, if in fact this is what is going on, other than himself. You can't. You can only control YOU and your reaction to his behavior. Here's a big hug and hoping that you can have the strength to explore what is going on, set boundaries, and detach if he is into drugs again.
I would tell you to get him drug tested, but the tests are so unreliable. Things like spice don't even show up, yet t hey are VERY dangerous.
Good luck. Sorry you had to join us, but we are here strictly to support you in your journey.