Praise be to Allaah.
If a person finds himself in a country where the people have already started fasting, he must fast with them, because he must follow the ruling of the country he is in. The Prophet (peace and blessings of Allaah be upon him) said, “Fast when the people fast, break your fast when the people break their fast, and offer your sacrifice when the people offer their sacrifice.” (Reported by Abu Dawood with a jayyid isnaad; there are corroborating reports narrated by Abu Dawood and others).
Suppose that he moved from a country where he had begun fasting with the people there, and went to another country. He has to either break his fast or continue fasting according to the ruling of the people in the country to which he has travelled, even if they end their fast earlier than the country where he started fasting. But if it turns out that he has fasted less than twenty-nine days, he must make up the fasts later on, because the hijri month cannot be less than twenty-nine days.