I'm trying to find a sixties short story of a young boy sitting at a typewriter and a host begins to type that he has to use the boy's body to find an escaped criminal from another planet. I keep thinking the name was Host. At the end the host cannot return to his planet and the boy invites him to coexist.
I would guess "Needle" by Hal Clement due to high similarity of plot structure, but (if I recall correctly) initial contact with the boy is via forming letters on his retina from inside his eyes (not via typewriter). See: