I’ve seen a lot of problems like this, including dealing with an alcoholic in my own family who didn’t “wake up” until literally waking up one morning in a psych ward in an area of town she thought was the local ghetto (actually, it has some nice houses there, including some pre-Civil War historical sites that I managed as a landlord at one point and it’s also not far from St. John’s Church where some guy gave some speech about liberty or death). The key point was waking up in a perceived ghetto and realizing she was being treated like everyone else in the ward.
The sad truth is people in addictive situations or who are in some form of denial, as is the case with your son, just have to hit bottom before they change. Bottom is different things for different people. For my relative, it was waking up in what she thought was the ghetto. The more trouble your son has financially while in QS, the faster he’ll be likely to hit bottom. If he has to come back to you, then he’s ready to listen to you. Just don’t take an “I told you so” attitude and use questions to get him to realize what’s going on instead of telling him.
For example, if he loses his apartment, ask, “But what about your friends? You’ve told me that they’re completely behind you. Don’t they believe in you and QS enough to let you rent from them for the time it’ll take before you can afford your own place?