I believe a year of service (with respect to the arb clock) is 172 major league days.
As farren83 and prezuiwf have pointed out, minor league assignments of less than 20 days count as major league service. So to clarify . . . he would need to spend 20 (or more) consecutive days in the minors. You cannot split it into two separate assignments of 10 and 10, for instance.