Well that would be really amazing if something like this happens in real life, but I really doubt his words because we all know that politicians promise us the things that we want and when they gain the presidential status then they know the art of U-turns. Many of the politicians have promised things and they just ignored their promises when they gained the powerful position in governments.
If he stick with his promise then I would really want someone like him to be the next president of America because a person who can back US dollar with Bitcoin can be a very valuable person for us Bitcoin enthusiasts. I also believe that anyone who is supporter of Bitcoin would support such candidates even with closes eyes because our main aim is to make Bitcoin more useful and if someone backs world's most acceptable currency with Bitcoin then we will support him.
But, if such people continue taking U-turns then our hopes will be reduced and we will have to wait for a honest and generous person who could make such a dream as a reality. Until then we will see that what would Robert F. Kennedy Jr do when he gets accepted as the next president of America.