starcrossed,
Been there done that. I bought a cheap breathalizer off of eBay and did the same thing you are doing. I blew in it and it showed no alcohol. He blew in it and it showed that he had been drinking. He SWORE that he hadn't. I spent nearly a thousand bucks on an expensive one like the state troopers used. It showed the same thing. That was when I realized that I had been lied to over and over again. All of a sudden things that had happened made sense. I felt like a fool but at least I did finally know the truth.
I hate to say it but you're being lied to. You said this man is your boyfriend. Don't make the mistake that I did and marry him. It won't get any better it will just get worse.
After I had finally had enough lies and insanity I filed for divorce.
Good luck to you.