Issei went to Hawaii to chop sugar cane in the fields of the "Big Island" and joined the Hawaiian National Guard at age 17 lying about his age to gain permanent residency even if the laws at the time disallowed him citizenship.
His high intelligence could've meant an instant admittance to the Imperial Japanese Army Academy at Asaka, Saitama, straight out of middle school.
He ran away from home and bought passage aboard a Pacific Far East Lines' tramp freighter headed to the port of Hilo, Hawaii. He nominally converted to Anglican to get a missionary visa to the Hawaii American territory. Though he probably could've just snuck in and no one would have questioned him since it wasn't the mainland.