This is a surname used as a first name - very common all over the English-speaking world. It's traditional in my end of Africa (not his) to use an English name as the first name and put the traditional name in the mn position; then the English name, more familiar to more people, is the default name outside the home, but the family and friends will regularly use the African mn.
Nixon is Nick's son - son of some long-ago Nicholas. Which you'll find by clicking on the blue hyperlink!
It's quite likely that his parents chose the name to honour someone or other - could even be the former American president, or someone they knew.