Can someone familiar with visas, US Customs and ESTA waivers help me (it's very urgent)?

My boyfriend was cleared, for the second year in a row, for an ESTA waiver to enter the United States. Then, in the middle of the night last night, five days before he’s supposed to get here, he got an email from US Customs saying that something was wrong with it.

We can not figure out if it’s some stupid glitch in the system, if US Customs is being paranoid because of what happened in Belgium or if he’s actually been flagged for some unfathomable reason.

Worse, he can’t get anywhere, with anyone. He’s gone to the US Embassy in London, he’s tried calling US Customs, he contacted Foreign Affairs in London and none of it is any good.

He hasn’t been given a reason for any of it and you automatically waive your rights to appeal when getting an ESTA waiver, but this is absolutely crazy. Last time he was here, he followed all of the rules, he’s never been arrested, he has no history of anything.

They keep saying the only option is to apply for a full visa, but he obviously couldn’t get one in time to come here. On top of this just being absolutely ridiculous, he’ll also be out almost $3,000.

Does anyone know of any option? Somewhere he can call and actually speak to a live person?? We’re both pretty desperate at this point.