So you purchase a breathalyzer for his car. He doesn't want one, and isn't required by law to have one. You "want" him to have one. He doesn't have to use it. Do you force him to breathe into it? Can you guarantee someone is with him every time he drives to make sure that happens? Can that person force him to breathe into it? If he does (which he won't) and it's positive...now what?
Bottom line...privately purchasing a breathalyzer won't stop him from drinking. If he continues to drink and drive, the best you can do is call the police if he drives away drunk. Until he gets real about his addiction, there is NOTHING you can do to stop him. What you CAN do is get help for yourself. Find an AlAnon group near you, and go to a meeting. Get support, way more helpful than wasting money on a breathalyzer that he won't use.