Thank you for bringing your question to Just Answer!
You don't have much choice about giving him space, as he's broken off the relationship. He's made a choice that drinking and drugs are more important to him than being in a relationship.
He's old enough to realize the consequences of such a decision, but is choosing to make it anyway.
Usually when people go down this path, the only thing that will turn them around is consequences. Trying to save or protect him from getting hurt is not in his best interest, because 1) you can't save someone who doesn't want to be saved. 2) if there aren't any consequences of what he's doing, there's no reason to stop.
You note that, at 19, he was "aways a smoker/drinker", which says he started using substances at a pretty young age. You may be seeing the progression toward addiction/alcoholism. Choosing substances over another human being is a pretty big red flag.
Trying to control him or his behavior won't work. If he becomes apathetic toward your relationship, that tells you it has run its course. Very few teenage relationships make it into adulthood. It's impossible to predict how long it will be before he chooses people over substances--and if he has a predisposition toward addiction, it may be many years. Based on the normal psychology of men, it would be unlikely for him to return to a past relationship--unless it seems like a challenge. Men like competition and challenges: that's why being too accommodating doesn't work with them. It may seem counter-intuitive, but your best shot at getting him to miss you is for you NOT to miss him. Read Temptations of the Single Girl and/or Why Men Love Bitches: From Doormat to Dream girl http://www.amazon.com/Why-Men-Love-Bitches-Relationship/dp/1580627560/ref=sr_1_1?s=books&ie=UTF8&qid=1322762333&sr=1-1
The only productive thing you can do at this point is work on detaching from him. It would be great for you to read Melody Beattie's books on co-dependence to help you through this time. Here's a link: http://www.amazon.com/New-Codependency-Guidance-Todays-Generation/dp/1439102147/ref=sr_1_5? s=books&ie=UTF8&qid=1322762430&sr=1-5
Re-building your life without him will either make him realize how much he misses you or it will get you started on a new path that isn't dependent on someone who has treated you this way.