The relationship between the Jagan and Mali in The Vendor of Sweets is not a very strong one.
Part of the reason for this is cultural. Jagan represents the traditionalist values of Indian identity. He wears homespun cloth. He looks at the wall that features a picture of the Goddess Lakshmi, who controls wealth accumulation. He tries his best to fuse his way of life with strict Gandhian principles of adherence to spiritual truth. Jagan carries with him the legacy of being a freedom fighter in India's struggle against the British. It is for this reason that he cannot easily surrender his past and what it means. The struggle for those values were real and represent priorities that are dear to his heart.
Mali does not share these values. He is different as he embodies much of the youth in India. He fails to acknowledge Indian/ Hindu traditions that he sees as "backwards." He sees his life as his own and not linked to anything larger. It is for this reason that he sets his mind to going to America, taking money from his father without consent to do so, and setting up abroad. On a personal level, the relationship between father and son is frayed because Mali blames his father for his mother's death. The differences are stark when Mali returns back to India. He feels that the country of his birth is foreign, as “He seemed to cower back and recoil from the bright Indian sunlight.” He has returned for financial fain and nothing more.
The generational gap differences between father and son help to contribute to a frayed relationship between them. The communication gap between them is even more difficult to overcome. Jagan cannot bring himself to effectively speak to his son, while Mali did not even write the letters that his father brandishes about with beaming pride. Their relationship is shown to be incapable of overcoming the intense differences between them.