I'm a EMT, Will my boss fire me if he finds out I have an arrest record?

When I applied for a job as an EMT, the application asked if I had ever been convicted of any crime, other then minor traffic offenses. I answered no. But I did omit(didn't tell them) that I had been arrested on suspicion of DUI 6 years earlier. I was never convicted and never charged with a crime, no action was taken against my drivers license or me in anyway. In fact I have a letter from the DA stating they never have and don't plan on prosecuting me. However an arrest record still exists and can be seen on a FBI or thorough background check. The background check my job did on me was not that thorough. I'm entering a paramedic program in a few months and a FBI background check will be done, which will definitely show the arrest. EMS is a small community and my boss will definitely fine out. How should I proceed? should I tell him now? should I wait until he approaches me. Should I hope he doesn't find out. I am going through the process of getting it expunged before the background check is done, however I don't believe it will be done in time. any advise would be great. I live in the US in an "At Will employment state"