The US embassy website states:
"Routine visa applications take approximately 5 workdays to process after the visa interview. Applicants who tick yes to box 38 should be aware that it is difficult to predict how long the application will take to process, but can expect to wait a minimum of eight weeks before being advised of the outcome of their application. Applicants who are advised at the time of the visa interview that their application will require additional administrative processing should allow at least 60 days for this stage of the application to be completed. Please note that the processing times quoted are approximate and cannot be guaranteed."
(The 'minimum of eight weeks', quoted there, is an improvement. Up until recently, the figure was 14 to 16 weeks)
'Box 38', above, refers to the series of questions which starts with "Have you ever been arrested .. ."
There's no way that the US embassy will know about your boyfriend's arrest unless he's already told them. The reason I say this is that, if he's attending an interview, he should have already completed form DS-156 online. If he's already ticked 'Yes' to question 38, it might be difficult for him to explain why he's now changed his mind. If he's not told them, it would probably be best to keep quiet about it. Even if he has ticked 'Yes' on the form, a little white lie might be in order. (e.g. "I ticked that because I wasn't sure. I was once asked to go to a police station but I wasn't actually arrested").
My feeling is that it might look a little odd if he didn't turn up for the interview. However, even if he doesn't, there's no way that the US immigration authorities can find out about his arrest.
Chris