Is the son sinful if he leaves university studies that he dislikes and seeks to pursue Islamic knowledge instead, despite his father's anger and the waste of his money on his previous studies?

A child is not obligated to obey their parents in studying a specific major. However, since the father has spent money and the son has come a long way in his studies, it appears that obedience and completion of the studies are obligatory, due to the father's vested interest in it. Sheikh al-Islam Ibn Taymiyyah said: "A person is obligated to obey his parents in matters that do not involve disobedience, even if they are disobedient [to God]. This is the apparent meaning of Ahmad’s general statement, and it applies to matters where there is benefit for them and no harm [to the child]. If it is difficult for him but does not harm him, then it is obligatory; otherwise, it is not." Obedience to parents is a means to Allah's pleasure, as Allah Almighty says: {And whoever fears Allah - He will make for him a way out And will provide for him from where he does not expect}. The student can utilize their free time, especially holidays, to acquire Islamic knowledge.