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Everyone entering the United States legally needs a passport. If someone is coming into the U.S. from a country that is not "visa exempt" (part of the "visa waiver program") then he/she would need a visa.
Citizens or nationals of the following countries* are currently eligible to travel to the United States under the VWP, unless citizens of one of these countries are also a national of Iraq, Iran, Syria, or Sudan.
* (England, Northern Ireland,
Scotland and Wales)
As Kenya is not part of this list, he would need to get a visitors visa to be able to visit the US.
You can find out more about that here: http://nairobi.usembassy.gov/non-immigrant_visas.html
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