He has to. By law.
http://blog.constitutioncenter.org/2014/02/article-ii-the-executive-branch/
Article II, Section 1, Clause 7: Compensation and Emoluments
Text Explanation
The President shall, at stated Times, receive for his Services, a Compensation, which shall neither be encreased nor diminished during the Period for which he shall have been elected, and he shall not receive within that Period any other Emolument from the United States, or any of them.
To preserve the president’s independence, Congress can neither raise nor lower the president’s salary during his term. Nor can a president accept any other pay.
http://blog.constitutioncenter.org/2014/02/article-ii-the-executive-branch/