What is the reason for Ali to become a radical, fanatic muslim?
On some levels, Ali experiences a disenchantment with his Western life. Notice in the opening lines of the story how Parvez comes to understand that something "is wrong" with his son. The possessions he used to value are all disposed of, the room that used to be the symbol of his privacy and personality is transformed, and the British girlfriend and his friends are no longer present. Each one of these reflects that there is a disengagement with "the West." Materialism that is so prevalent in the West has been rejected, the notion of individual privacy and subjectivity that is so much a part of Western culture has been abandoned, and the social attachments that dominate Western culture have been repudiated. In these, it is evident that Ali is experiencing a disconnect or displacement with the West. I think that such a perception of reality compels Ali to embrace a set of values found in Orthodox Islam that are entirely separate from Western constructs of the good. I would have to disagree with the characterization of Ali as a "fanatic" and "radical." The greatest part of Kurieshi's story is that he constructs Ali and Parvez to be on opposite ends of the political spectrum, yet it is the Western father who embraces the ends of terror and intimidation. Ali does not. While Ali might fulfill the stereotype of the terrorist, it is Parvez who acts with complete disregard for the life of another. In the end, Ali might be a victim of his father's (and our) stereotypes.
Obviously, Ali is disappointed of his father's life, because he cannot see any sense or worth in living it. For Ali, Parvez only seems to live for "Western" pleasure (alcohol, prostitutes...), he does not see any philosophy of life behind Parvez's lifestyle. Ali simply does not understand his father's relationship to Bettina, for example.
Ali simply needs a perspective in life - and he has found one in religion. Praying and feeling solidarity to a big Muslim community comforts him and the strict rules give him a structure for life he can follow.
In my opinion Ali disliked the life of his father. He didn't like the fact, that his father meets with a prostitue, because Parvez is married. Ali also disliked the father´s consumption of alcohol. I think Ali didn't want to become a man like Parvez and because of that, he changed. He became the opposite of his father, a man, who follows all the strict rules of his religion and culture.
I can just hardly imagine that Ali became a radical, fanatic muslim by himself, i would be surprised if he developed such hatred from one day to the next, just because he recognized one day how bad the western society is. Perhaps he got in contact with other muslims that showed him the bad sides of the culture he lives in and stimulated him to develope hate so that he later was only fixed on his faith.