Hi Cellar. It's been a rocky relationship.
Two things stand out in what you have shared:
He says he believes having me in his life again will be another inspiration to keep him on his better path
You then said to him that you would have to be considered as well. That should go without saying but it's good that you did say it because this statement above seems very self-centered.
The second part and for me this would be the big, big! waving red flag:
Then we hooked up and he ghosted me because he was ashamed over how he acted and felt that his motives weren't right.
You can call it ghosting but as horrible as the connotations of that are, he just decided to ignore you, give you no explanation, show complete and total disregard for your feelings and just leave you hanging there feeling horrible.
This was 4 months ago - 16 weeks! Now he has had this huge revelation and he is all zen and together. Frankly I'm not buying it. Now that is just my personal opinion but I hear the doubt in your reply too.
You have known him for 15 years - as HL said, no rush here. How about you review in say another six months when he has a year of sobriety? Even then, you might want to take it slow.
In the meantime, how about being friends (not with benefits, because as soon as you add that to the mix there is all the emotional/hormonal bonding).