I'd put the stop on the check immediately, not wait for him to give you the check back because he's probably "lost" (cashed) it.
It is a terrible shock to have to confront the evidence of someone with an addiction, and I am sorry that you are in that situation.
You sound very levelheaded, and you're making wise choices for yourself. There is so much support here on Sober Recovery, and I encourage you to join us as we sort out what it is about us that let us ignore the red flags for so long. That's the recovery process for partners of addicts.
ShootingStar1