I don't think there's anything wrong with you asking him to take a drug test... It was your choice to stick with him when he relapsed and had a tough time before, so you deserve to know whether you're about to go through that same hell again.
I asked my ex to take one, and he said no because he was "proud" of how well he was doing, and that was good enough for him (only to find out later, he said no because he was using again..). I asked him again at a later date and he took it, then tried to make excuses for why it came up as positive, until he finally admitted it.
I don't think it's your codependency acting up, I think you reserved the right to know once you decided to give him another chance. What you do with the result (which is really only if he admits it) is up to you! Be true to you