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.