Thank you so much everyone for your feeback so far. You're all right. It was far too sudden and has left me floundering for some type of grounding.
Originally Posted by
outtolunch 
Rehabs do not cure addiction. Best case, a rehab can teach a highly motivated guest some of the tools of recovery. The real hard part begins after rehab. Generally speaking, it's usually better for everyone, when a former rehab guest tranisitions into a sober living enviornment instead of returning home.
Where's home for this guy?
There is nothing you can do or not do that is going to keep him clean or cause him to relapse. You are not that powerful. None of us are.
Consider taking the time to figure out your own boundaries for this or any relationship. A boundary begins with " I will/will not..." Examples of boundaries include:
I will not be in a relationship with a liar.
I will not be in a relationship with someone who steals.
I will not be in a relationship with a criminal.
I will not be in a relationship with a drug addict.
A boundary is your bottom line. We are responsible for enforcing our own boundaries on ourself. If/when we find ourselves in a situation that challenges our boundary, we take responsibility for ourselves and remove ourself from the situation.
Many confuse boundaries with attempts to control other people. Most attempts to control someone else begin with " You will/ will not...or else..." We have no control over other people. The more we try to control other people's behaviors, the less time we spend controlling ourself and our own reactions. And that's the only thing we do control.
There's absolutely nothing wrong with taking a break from this relationship and figure out what's important to you.
I'm sort of confused about this, because all four things you listed WERE my boundaries. And as far as I know, he didn't fit any of those until he started using, which I was also ignorant of until a month ago...
This is where I run into problems. At the rehab they insisted to us that addiction is a disease, and they only did these things as a result of the disease. That it wasn't really them, just symptoms.
I don't know how to apply the boundaries I've set... if those actions were only symptoms of his "disease" and not him.
But maybe I'm just looking for excuses to stick around? I really don't know. Because up until I found all this crap out we were only a few months off from buying an apartment together. The realization that the person you thought you were closest to is a drug addict is staggering. I'm completely astonished that somehow he could hide that from me with the amount of time I was around him.
Oh, and to answer the question. He is going back to live with his parents. Which seems like a mistake to me. They're controlling and enablers. I have repeatedly asked his mom if she'd like to come to al-anon with me, but she always has an excuse. All I can do is offer, and hope she eventually realizes she really needs the support as much as I do :/