An ex-business manager for R. Kelly and his co-defendant at the singer’s federal trial in Chicago expressed doubts on the witness stand Thursday about Kelly’s insistence in the 2000s that he never sexually abused minors — testifying a day after the former employee told jurors he’d had no reason to doubt his boss was telling the truth.Derrell McDavid’s testimony, which could be a major blow to Kelly’s hope of acquittal, came at the end McDavid’s second day on the stand. He and the …