The story is told from Parvez's limited point of view. Kureishi doesn't tell us what is going on at first because he wants readers to experience the same confusion that Parvez does about his son's strange behavior. This builds suspense: like Parvez, we are mystified and want to get to the bottom of why Ali is giving away all his goods and otherwise behaving oddly. More importantly, Kureishi wants us to understand how out of touch Parvez is with his son.
This is a story about generational and culture clash. Parvez, who grew up in Pakistan and knows how harsh life was there, is nothing but thrilled with the freedom he experiences living in London. He can't conceive that his son would be anything but grateful for the life Parvez offers him. Parvez can afford to buy him a computer and pay for him to go to college. His son's rejection of these gifts is confusing to Parvez and his friends, as it likely is to readers who share the same Western worldview as Parvez.
Because we view events through Parvez's eyes, we are likely to sympathize with him. We share his shock as he realizes his son has become, in his opinion, a religious fanatic. It is only over a dinner that the two have together that we begin to see that Ali has reasons to criticize and reject his father, such as for his excessive drinking.