The first idea that the President might be given the power to adjourn congress appears to have originated in the five-member Committee of Detail, consisting of Oliver Ellsworth, Nathaniel Gorham, Edmund Randolph, John Rutledge, James Wilson, and can be found in handwritten

amendments made by Rutledge. https://www.quillproject.net/session_visualize/3347#77519 The provision was never substantively discussed, and two possible explanations for its addition seem credible. The first is that both houses of congress were prohibited from adjourning without the permission of the other,

presumably to stop adjournment being used to frustrate the workings of Congress. However, the power of the President to settle disputes between the House and Senate over the question of adjournment was added to a boarder section listing the president's powers, and may also

have been part of efforts to make the presidency appear a more substantive role. Perhaps for that reason, though, when this paragraph was later discussed in the full sessions of the convention, it was the other powers (such as those of pardon) that attracted debate and comment.

The full history of the phrase "he may adjourn them" can be found by following the links here. https://www.quillproject.net/jumpTo?quillGoTo=ph%3Ahe+may+adjourn+them The broader powers of given the president proved controversial, and the relevant paragraph debated several more times.