There is no published amount. It is totally up to the interviewer. Some have reported being approved on $40k-$50k. Less may work too. He just has to try it. He brings bank or financial statements to prove his money. There is no form to fill out. If he has money in several locations, then an organized spreadsheet summary along with the financial statements would be good.
If your parent is sponsoring, your fiance takes your parent's I-134 and proofs to his interview. London does not need to see a form saying you make $0.00. They accept a sponsor other than the fiance. You (fiance) do not have to sponsor or fill out anything if your Dad sponsors. Read the replies from the people who know London best (Post #2 and #3).
