What should a man do who doubts the obligation to make up a day from last Ramadan, knowing that tomorrow is the first day of the current Ramadan?

If a Muslim doubts the validity of a day's fast from the previous Ramadan, he is not required to make it up (qada'); because the default assumption is the validity of the fast, and one does not deviate from this default unless there is certainty. However, if he doubts the very act of fasting for a day in Ramadan, for example, if he doubts whether he completed the month's fasting or not, and his doubt was not incidental after finishing the fasting, then he must make up what he is not certain he fasted; because the default assumption is that he did not fast it, and his liability is not cleared except by certainty. And if he delays the make-up fast until the next Ramadan begins without a valid excuse, then he must make it up along with an expiation for the delay.