"Sure, dear. I'd be happy to go talk to a counselor about your drug issues and how it affects my trust levels when I've caught you lying and my anxiety about co-parenting two young children with a drug user. You go ahead and make the appointment. Tell me when. I'll be there. Good idea!"
I don't know how the users in our lives know with pinpoint accuracy what things to say that will hit us in our most vulnerable places - maybe they carry the info they learned from when they were not using and we used to have real conversations and shared hopes and fears. I guarantee if you call him on this and keep asking when your counseling appointment is, you'll hear that he's changed his mind or that he doesn't need it, only you do. In which case you can say, "Yep, maybe I do need to talk to someone about our issues - drugs and trust. Okay. I'll go. You pay for the babysitter."