What does it seem like he does with his free time if he has no interest in hobbies/activities? When you are home what's his behavior like? Does he tend to isolate himself away from people for long periods of time?
I'm going to have to second the crystal meth theory. And as an addict, I would never admit to my using to anyone. If your son is living with you have you considered making him take a drug test? They sell them at local drugstores. If you do this, don't tell him ahead of time.
I would also wonder about meth, although there are others that can cause this type of behavior. It could also be mental health related. If he's denying any drug use, I would wonder if he might agree to an assessment by a mental health professional. Hopefully that might get to the root of what's really going on, whether it be mental health, drug use, or a mix of both.
Wishing you the best,
Things are a little slow here nowadays so you it takes a bit to get an answer. Unless your son is just mentally ill it sounds like crystal meth. I've been around users and they become paranoid, don't sleep, say things that don't make sense and lie aggressively. I wouldn't think it would last days after last use though. Are you sure he isn't secretly using? It's pretty easy to hide and it does last a long time. If he could use once a day that would probably do it.