If it truly does fit in the shoulders, you should probably see about having it taken in some across the middle. If you were to look at the average PC, I think many of them could be made to meet across the owner's stomach, even if they aren't made to button. But you are talking about an overlap of several inches, it sounds like. I would hope a good tailor could remove some of that excess without ruining the lines of the jacket, but it sounds like a job for a skilled person.

Can you post a picture so that we can see how much too big it is?