Is a son obligated to obey his father in working on his agricultural land, given the son's engagement in other work, his studies, and his commitments? And would he be sinful for not working with him?

The right of a father over his child is immense, and obedience to him is obligatory in what is good and lawful. This includes spending on him and serving him when needed. If the father is self-sufficient, then it is not obligatory for the children to spend on him or work on his farm as long as he is capable of hiring someone to do so, unless they volunteer. Therefore, it is not obligatory for you to abandon your work, studies, and family interests to work on your father's farm. However, you must show him filial piety and treat him with kindness, for filial piety towards a father is among the best ways to draw closer to Allah. "The Lord's pleasure is in the father's pleasure, and the Lord's displeasure is in the father's displeasure," and "The father is the middle gate of Paradise." Strive to please him with what you are able to do and what does not harm you, and apologize to him gently and politely for what you are unable to do or what would harm you.