What is the ruling on someone who swore to stop watching pornography and fasted for three days, then returned to it; should he fast for another three days or pay expiation?

You are obliged to sincerely repent from the forbidden act. The expiation for an oath is to feed ten needy persons, or clothe them, or free a slave. One should not resort to fasting except after being unable to perform these three. If you fasted while being able to feed, then the fasting is not sufficient, and your obligation is not discharged. However, if you fasted while being unable to feed, then your oath is dissolved, and no further expiation is required of you. Nevertheless, you must abandon this reprehensible matter in obedience to Allah the Almighty.