Welcome to the Board.
I have to confess that I did not read your post in its entirety. I've seen this movie often times to generally know the plot line of "boyfriend/girlfriend is ignoring me after rehab" goes. So for better or for worse, here's my take.
There's two possibilities. The first is rehab scared the daylights out of him enough for him to prioritize his recovery. If this is the case, it's understandable that he creates some space between you because he doesn't have the bandwidth to be an available, accountable partner in a romantic relationship. The second is you two are done but he hasn't explicitly told you this. In the end, the first scenario and second scenario are ultimately the same in terms of what you have to do, which is to respect his decision, even though you hate it and it hurts.
Being under the influence of narcotics suppresses the true nature of the person. This means that while he was using, he was not experiencing your relationship in the same way you were. You had all your wits and faculties about you. He, on the other hand, was compromised. In order for him to be a reliable romantic partner, he has to embrace recovery: not using, going to whatever program that works for him, changing his behavior, etc. That's going to take time and an enormous effort on his part, and there's no guarantee he's going to be successful. The best thing you can do -- for you -- is to decouple and allow him his freedom.
And yes, doing that will hurt, and suck, and I get it. But it's better than holding onto someone that, in the end, will only hurt you.
Read as many posts as you can here. Read them slowly. See what they all have in common. Learn what all of us here have done to regain our sanity. And in time, you're going to get through this and be OK. Trust me on this...and if it helps, read my posts to see where I was in January 2012 to where I am at the end of 2017.
Be strong.