Premature Ejaculation

My husband and I have been together for over 2 years and for the past 6 months maybe, he has had PE and it keeps getting worse. We have tried the start-and-stop method; does nothing.. He has tried thinking of other things during sex; does nothing.. He has tried masturbation before sex; does nothing.. He has tried taking herbal vitamins; does nothing.. He says he's not stressed (not to mention this has been a long-term issue).. And we have been together for 2 years, so it's obviously not him being nervous. Have any of you ladies' husbands had the same problem, and if so, what was the cause and solution? Please help!