What was Jim's father about to tell Jim before the wedding?

After the scene in American Wedding where Jim thanked all bestmen, Jim's father was saying "Son, I think it's time to tell you" and then Jim said "I know". And the scene ends with the two of them walking forward.

What was Jim's father about to tell Jim? It almost feels like somesthing surprising or important, like "you were adopted"... But there's no hint anywhere from the movie or from the American Pie series what Jim's father was about to say.