A marketing plan is the document that puts strategy into tactical operation.
Typically a marketing plan
a) spells out the mix of products that will be offered to different customer populations b) what costs will be expended
c) what results are expected both in terms of sales and profits
The marketing plan also details control and test procedures to make sure the plan it working to maximum effect. These control and test procedures also identify problems in the execution of the plan so they can be corrected quickly.
The best way to make some future event happen is to spell out the way to make that event happen in writing and have everyone in the company 'buy in' to the written document. My business includes a lot of management advisory services and we regularly start with a simple revenue budget and the way to make the revenue. It's amazing how many companies can meet a plan once it's written down. A marketing plan takes the simple 'write it down' equation and enhances it with predictions on demand, price, production, and sales to create a complete across the company plan for every aspect of business.
Since the formal marketing plan is the best way to insure company growth and success I'd convince my supervisor to engage in the marketing planning process as a way to both keep and build his job.