I think that one could say that Rakesh did remain devoted to his father and his family until the end. Rakesh never really changes. He is shown to be a devoted son at the start of the story and this is how he is at the end. He acts and embodies the reverential son throughout. I think that there are two ways in which to see this. In one respect, Rakesh fulfills the responsibility of children in the traditional expectations of Indian society. He does his parents' bidding. They demanded that he respect their word and be obedient. When Rakesh needed to come back to India, he did. When he needed to marry a woman not of his own choice, he does. When he is expected to adhere to what social standards expects of him, he does. Hence, this makes perfect sense that when he is needed to cure his father, he does not shy from this and pursues it vigorously and with devotion. In this light, he does represent a "devoted son."
On the other side of the coin, perhaps the ferocity and intensity within which he treats his father is a reflection of all that he has had to sacrifice because of his devotion. Rakesh is not shown to be one who has a voice at all. He does his parents' bidding. Perhaps, it is for this reason that he dominates his father with so much zeal in the hopes of curing him. It would simply make no sense for Rakesh to have given up his life for his parents and his father, in particular, only to see him die. It is here where Desai might be showing that the demands placed on children, if they are not monitored to allow some level of voice and acknowledgement, devotion can be brought out in other ways. The dominating aspect in which Rakesh relates to his father could be more of a reflection of his own dissatisfaction with having lived his life for his parents and the inability to accept the reality of age and death could be his response to such a condition. It is here where Rakesh's devotion could carry another side to it. Regardless, I do think that Rakesh's devotion is present in the story.