When must an employer announce his intention to match employee contributions? Must he do this prior to the Plan year or can he wait until the plan year is over and then decide if wants to make matching contributions? The plan document states that employer contributions are discretionary.

At the end of the plan year is fine. This is going to act just like a discretionary profit sharing contribution, but the money is only allocated to individuals that deferred.

Discretionary match done that way defeats the ostensible purpose of the match. If the match is just a device to enhance owner benefits, it does not matter.