On the positive side, it sounds like he's really doing well with staying sober. No drinking, a job, and active in the program. You can't argue with those stats.
However, it sounds like perhaps he still hasn't "figured himself out", for lack of a better term. Often times, for people like this, marriage seems like a good idea. If a man still is having problems with his life, he might think that a wife will automatically fill those holes. Sometimes women get married for the exact same reasons (heck, maybe more often). Things not going perfectly in life? Get married, and all of your prayers will be answered!
In my case, when my alcoholism was at its worst and things were taking a downturn....I went and got a ring for my girlfriend! I wasn't happy in life, and thought perhaps this type of commitment would set me straight. Now, luckily I never proposed (I had a little common sense left). I definitely saw marriage as a "life raft" for my situation and I knew that wasn't right.
Alas, it sounds like maybe he's planning on asking you for the wrong reasons. The right time for a man to ask is when he is feeling strong, secure, and interested in adding someone to help him grow. But if he is still struggling, he might be asking you because he feels that putting a ring on your finger might solve some of his problems. If that's the case, you might be best off saying "no".