Can you be charged with a DUI if you were not operating the vehicle at the time but were sitting in the driver's seat?

A person can be arrested for a DUI even if they are not driving at the time (even if they are not behind the wheel at all). Legally all that is needed is evidence of driving. For example, if your boyfriend was behind the wheel or had the car keys in his possession; that could be considered good enough evidence and legally sufficient to demonstrate that he exerted control over the car. Further, I don't know what, if any, tests were administered to him at the scene (breathalyzer, blood, field sobriety) which would further legally establish that he not only exerted control over the car but did so at a time when he was "under the influence" (legally intoxicated).

That having been said, there are defenses can be enlisted to fight this DUI which could result in getting the charge reduced or dismissed outright.  At this point your boyfriend needs to consult with an attorney that handles these type cases. Since a DUI triggers both civil and criminal cases, he really should have legal representation. He can be left with a criminal record, as well as lose his driver's license and have his insurance rates affected. Additionally, this could even impact his employment and eligibility for professional licenses, etc.