I don't know your history with this guy but is it possible he's just telling you that he quit using as a way to make you feel bad?
Sort of like, "Well look what you're missing out on....now I'm clean and sober and not with you"
I wouldn't be so sure that he's actually clean...rather....he might just be saying he is so that you'll feel even worse about him leaving and want him back because he's "clean".
Am I making sense?
In any case....hang in there....*hugs*