Trust your gut. Something is up. My guess is either a mental illness such as bi-polar or Adderall/Vyavance abuse. Or both. I am not a doc and have only a tiny bit of experience with this stuff but my alarm bells are going off. As yours are. Act on it.
I have no experience with this, but I would not delay in confronting him about it before he turns 18 and you have no legal power to do anything. I also would snoop in the backpack and search his room. If you find something, you may well save his life or at least his future. If you don’t find anything, there’s no need for him to ever know you did it.
This didn’t involve a suspicion of drugs. But I did know something was really really wrong with my D. I sat her down and said “you know that I love you and your sister more than anything. I want you to know that there is nothing-NOTHING-you could do that would ever make me stop loving you. I want to know what is wrong so that I can help you. Nothing you tell me will change that. Let me help you.”
OMG, the dam burst, and after a lot of sobbing and hugs, we got straight to work on the problem. She told me what I said made the difference.
Maybe some version of the above can get a dialogue going. If he won’t talk, I’d go to plan B, which may well involve invading his “privacy.”
I have no experience dealing with people abusing Adderall, but I do in with dealing with drug abusers (cocaine, pills) in general.
Drug abusers go to the bathroom often. They are moody - but so are teens, in general, so that one may or may not be an indicator.
I also say - Trust your gut. Go through his stuff. Take him to see his doctor.
But, also - can you just ask your son directly, tell him your concerns? Ask him if he would empty his backpack out for you. Ask him about why he checks it so often, and why he goes to the bathroom a lot. Tell him you still love him, no matter what, and you will support him if he needs help, and do whatever it takes to see that he gets that help - even if he doesn’t want it, himself.
We used to look at his eyes. Here are some other suggestions: http://www.narconon.org/drug-abuse/signs-symptoms-of-drug-abuse.html
A tip I learned was to have a phrase ready when you do hear troubling news.
Knowing that your first words can be “I love you, and we will get through this together” will not solve the crisis, but it tells your loved one that they are not alone, and that you are supportive and willing to help figure it out with them.
So think up a phrase you can use with your child when the truth does come out, and maybe it will set the tone for an open and honest conversation.
Hi folks, as the OP, just wanted to let you know that 1) these messages of support literally make me want to burst nto tears. At the office. ugh. But I don’t think I realized how scared I have been until I wrote this whole thing down on paper and 2) the practical advice i’ve received here has been greatly helpful to me. Now I need to walk into the lion’s den, so to speak, and confront this head-on…
I’ll update all here if I handle this the right way…but I’ll likely disappear if i screw it up. 
If you ‘screw it up’, you’ll likely need even more support. So don’t disappear, let the caring, faceless strangers be of support.
Best of luck, Firefly
I hope your son is just on some teenage boy hormonal roller coaster ride. But in case he is in some sort of trouble, if it were me I would prefer to have my son angry with me and healthy than the alternative. We are here when and if you need us. Take a deep breath, hugs and good luck.
Our oldest went through a personal crisis so we got him some counseling. The first time was a disaster - the counselor did not specialize with teenage boys (mostly middle-aged women) and did not tell him that what he said would be kept in confidence. He lied through his teeth to her, afraid anything he said would get back to mom.
The second time we, and he, did much better. The new counselor deals very specifically with his type of kid and started with the talk about how all of the sessions would only be with him, no mom or dad allowed. It (12 weekly sessions is all it took) made all the difference in the world for him. Maybe someday, when he is ready, he will tell us what went on during those sessions. We don’t question it and are extremely grateful for the results.
My DD takes prescribed Vyvanse and she exhibits the behaviors you describe. Excessive talking and busyness as the drug kicks in and occasional exhaustion and zoning out when the drug wear off. Possession without a prescription can be a felony so it is not a matter to take lightly.
Hugs to you.
I think you should talk to him and tell him you notice that something is going on, that you notice his behavior has changed and you want to talk about it. I’m not sure I would give away the backpack/pocket thing, because if he won’t tell you you will have to snoop. Just that daily routine with his pocket and backpack and not transferring the item in front of you is suspicious. That with the babbling alternating with spaciness, plus the Adderall slip sounds pretty likely that he is using it from what you describe of the babbling contrasted with spaced out.
It isn’t something that is going to make you really high, like even strong pot or getting drunk would, but it does give a bit of a euphoric rush shortly after you take it, then a feeling of alertness an focus for hours. It can cause trouble with sleeping and eating because it is an amphetamine. And I suppose if you were trying to get higher from it in terms of a bigger rush of euphoria and took more, the symptoms would increase and you would have a feeling of ‘crashing’ after. If you were in the habit of taking it and had to stop you would likely have feelings of irritability, agitation and loss of focus. If you use amphetamines long enough, you can get so that you can’t experience pleasures without it. Enough use with sleep deprivation could be very detrimental and lead to paranoia. I’m not sure about going to the bathroom a lot as usually one of the effects, maybe he is talking a lot of water for dry mouth. I guess I would also be concerned that he would be trying to get high on other related and worse amphetamines like meth and think that the adderall seems harmless so the meth might too if he was someplace it was offered.
I take vvyvanse, but I don’t experience a " rush" or need to chatter at all. I suspect some of your kids are on too high of a dose.
I would take my child whom I suspected of self medicating, to a physician who could evaluate them and establish if there was something there to be treated, and to work out an appropriate treatment plan.