EDIT: I said father, I'm sorry I didn't see your screen name prior to posting this, looks like your his mom in that case, sorry! Same message applies
Best of luck to your son, he has a long road ahead of him. My most upmost of well wishes are with you both.
You've gotten some decent advice already, and sound like you have done enough research that you don't need much advice -- other than some reciprocation and reassurance you've received. So, I don't really have anything to add other than a reiteration of what another said already:
Buprenorphine, being the "drug" in suboxone/subutex, is in fact used off-label for depression by some psychiatrists with great results. The positive side effect being that coincidental use of a full agonist provides no reward, no euphoria, no sense. As his father (and Iam in no way trying to suggest how you SHOULD interact with your son), but maybe you could... step up, as you have every right, to just plain out get him put on buprenorphine for his own good. One of those things that I would classify as many moves my parents made when I was growing up for my own good, even though I strongly disagreed at the time -- and now in retrospect as a successful 30 year old, APPRECIATE SO MUCH. This is the type of parent-decision turned no-alternative action that could be the very best thing for your son. He will appreciate it probably sooner than later, and you can sleep more soundly. Eventually, he may very well tell you as a grown adult that it was the best thing you ever did for him. It won't end there, though, as you'll have to be an active participant in seeing that he doses it as he should everyday. Perhaps for you to dispense, watching him take it and dissolve under his tongue. He is at HIGH risk for relapse. This is a way to prevent that, and to treat his depression while he seeks therapy. And that does not mean a psychiatrist, as they don't care to talk about anything other than what pill they're going to throw at you next. That's NOT to say that he won't need some traditional anti-depressant medication treatment. My wife is on Celexa as well as Abilify, ever since -- she's been a different person. Depression lifted, she sleeps SO much better, anxiety is nothing but a whisper. She switched from JUST Zoloft, which was not really doing anything at all in comparison. Just a subjective experience to relate.
By the way, just FYI:
Suboxone is name brand and also contains an opiate receptor antagonist known as Naloxone that is "meant" to deter abuse of a full agonist opiate/opioid (heroin all the way down to hydrocodone and codeine). This is, for lack of a better understanding, just a play on ignorance as it is the buprenorphine itself that blocks other opiates due to it's own pharmacology. The Naloxone, when used sublingually as directed, is "inert" (as is advertised), but even when injected -- the buprenorphine blocks even the Naloxone from binding to any receptors in the brain. So, IMO: it was a neat way to get a patent. But again, not be discouraged by that, as it is the buprenorphine itself that blocks all other full agonists. Just doing a little informing.
Subutex is the brand name for the pill that contains ONLY buprenorphine and it is available as a generic, making the generic dirt cheap compared to suboxone. And by all means is the exact same thing after it enters the body as suboxone.
It sounds like he's done a poor job of making his own decisions to this point regarding what he's decided to put into his body, so his choice to go cold turkey may very well be again a poor decision. Perhaps it's up to you as his father to make the tougher, wiser decision out of love and knowing what is best for the most possible positive long term prognosis.
Then, sometime in the future, when he is at least a year old or maybe more - after he truly has gotten his life, priorities, goals, and motivation in check, he would know when the time would be right to taper down and remove himself from the medication. "Bupe" (as its referred to in short) can be tapered EASILY down to 2mgs and less (as low as .25mg without much discomfort), and can make this heroin withdrawal feel like a non-event (especially if it truly was no more difficult than you perceived it to be), albeit symptoms may last up to a week versus 3-4 days.