He is going to travel and will remain abroad for three months; does he have to fast Ramadan?

If a person travels to a place and decides to stay there for more than four days, then he comes under the same rulings as a resident according to the majority of scholars among the Maalikis, Shaafa’is and Hanbalis. So he comes under the same obligations as a resident with regard to fasting and offering prayers in full. 

It says in Fataawa al-Lajnah al-Daa’imah (8/99):

Travel in which the concessions for the traveller are prescribed is that which is customarily regarded as travel, and the distance concerned is approximately 80 km. If a person travels this distance or more, he may avail himself of the concessions of travel, such as wiping over the socks for three days and nights, joining and shortening prayers and not fasting in Ramadan. If the traveller intends to stay in the city for more than four days, then he is not entitled to the concessions of travel, but if he intends to stay for four days or less, then he may avail himself of those concessions. The traveller who is staying in the city but does not know when his stay there will end and has not set a specific time limit for his stay there may avail himself of the concessions of travel even if that is for a long period. There is no differentiation between travel by land or sea. End quote. 

Based on that, what you have to do is fast Ramadan and offer prayers in full during this period, unless you are travelling from city to city and the distance between them is 80 km or more, in which case it is permissible for you not to fast whilst travelling, and it is also permissible for you to join and shorten prayers. 

And Allah knows best.