Rather than surprises and ultimatums, you could inform him that you have home testing kits and if you suspect he is using, you will test him and if positive, you may request/insist he meet with a professional or discuss it with his PCP, etc. The delicate dance is giving him as much control over the process while retaining overall authority over his well being and related decisions. In other words, he may choose to get high, but there will be ramifications to that behavior.