Should he fast with his own country or with any country that sights the new moon?

You have to stay with the people of your country. When they fast, then fast with them, and when they break the fast then break the fast with them, because the Prophet (peace and blessings of Allah be upon him) said: “The fast is the day when you fast, and the breaking of the fast (al-iftaar) is the day when you break the fast, and the sacrifice (al-adha) is the day when you offer the sacrifice.” And because dissent is evil. So you have to be with the people of your country. So when the Muslims in your country break the fast, then break the fast with them, and when they fast then fast with them. 

As for the cause of differences, the reason is that some people see the new moon and some do not see it, then those who have seen the new moon may be trusted by others who follow their sighting, or they may not be trusted, and hence differences arise. One country may sight it and rule accordingly, and fast and break the fast on that basis, whereas another country may not be convinced of this sighting and may not trust the other country, for many reasons, political and otherwise. 

What the Muslims must do is fast all together when they see the new moon, and break the fast when they see it, because of the general meaning of the words of the Prophet (peace and blessings of Allah be upon him): “When you see the new moon then fast, and when you see the new moon then break the fast, and if it is too cloudy then complete the number of days as thirty.” If they are all certain of the validity of the moon sighting, and that it is real and proven, then they must fast and break the fast accordingly, but if the people differ and do not trust one another, then you have to fast with the Muslims in your country, and break the fast with them, in accordance with the words of the Prophet (peace and blessings of Allah be upon him):  “The fast is the day when you fast, and the breaking of the fast (al-iftaar) is the day when you break the fast, and the sacrifice (al-adha) is the day when you offer the sacrifice.” 

It was proven from Ibn ‘Abbaas (may Allah be pleased with him) that when Kurayb told him that the people of Syria had started fasting on a Friday, Ibn ‘Abbaas said: We saw it on Saturday, and we will carry on fasting until we see the new moon or we complete thirty days. He did not follow the sighting of the people of Syria because Syria is far away from Madeenah and there may be a difference in moonsighting between the two. He (may Allah be pleased with him) thought that this was a matter that was subject to ijtihaad. You have a good example to follow in Ibn ‘Abbaas and the scholars who followed him and said that you should fast with the people of your country and break the fast with them. And Allah is the Source of strength. End quote. 

Shaykh ‘Abd al-‘Azeez ibn Baz (may Allah have mercy on him).