That's not true. There may be a rule that says he has to win 5 states in an unbrokered convention. But in a brokered convention anybody can win, even someone who never entered the race at all.
http://www.gop.com/About/Rules31-41.htm
No mention of brokered or otherwise anywhere in the GOP rules. Im curious to read where it's displayed how the brokered convention even works.
(b) Each candidate for nomination for President of the United States and Vice President of the United States shall demonstrate the support of a majority of the delegates from each of five (5) or more states, severally, prior to the presentation of the name of that candidate for nomination.