The given situation is called as “Expectation Management”
Explanation:
"The management of expectations is one of the most powerful weaponry in psychological warfare. In managing expectations people instinctively disregard other people's thoughts and then use the technique intentionally, considering own ideas as they reveal them to other people."
The manager knows just well the essence of your venture, his point of view is a little different from yours and you have to understand his point of view (or even more) because he wants to comprehend yours.
Therefore, the manager is less involved in the execution and the technical aspects of operation than can the performance and how it works to meet its organizational goals. And that's how you should refer to the manager. Clearly, your ability to provide is limited, depending on how many hours you work every day.
Any requests he makes, create scope, calculate the cost (how fast), and ask him to give priority to other demands. On this basis, you can determine when you can provide what.