Hm, what made you think he had "disappeared"? Just the fact that he didn't respond to your letter?
I assumed someone at the program had told you that he left.
I think maybe you're expecting too much from someone who is in rehab. I'm not saying your feelings aren't normal, but they may not be realistic. Ever since he went in, you have sort of been on tenterhooks worrying about what he's thinking, what he's doing, what he's feeling, why he is or isn't doing this or that.
You get no guarantees. Even if he makes a ton of progress in rehab and makes a ton of meetings and works with his sponsor, there is always the possibility of a relapse. I have a feeling you are going to be stressing out once he gets out, too, watching for any signs that he might do that. Again, it's understandable, but it's going to cause problems for you, and for the relationship.
You are POWERLESS over his recovery. Period. He might make it, he might not. He might make it, and you won't want him anymore, or he won't want you anymore. Those are all possibilities.
Right now, so far as you know, he is working on himself. You need to have a life that will be good regardless whether he stays sober, and regardless whether you continue your relationship with him.
Keep the focus on you. That's the only thing that will ultimately bring you happiness.