That kind of sets you up in a position of being his parole officer/ keeper. Is that the role you want to play in a relationship? How will you feel if he agrees to take the test and it is negative? What happens next time you suspect he is using? What will you do if he refuses the test?
Continue to watch his actions, if he is using more will be revealed. I know that my gut instincts about my ex drinking were always right on, even if I didn't see him do it, even if he lied to my face. We have those feelings for a reason.
How long have you been together? How long has he been using? Is this his first sobriety attempt? Are you or he working a recovery program?
Agree with Hopeful about the face to face support for you- naranon, Alanon, celebrate recovery. That will be a great asset for you whether he relapses or not.