What is the ruling regarding parents refusing their son's marriage to his paternal uncle's daughter, whom he has wanted to marry for five years?

The questioner did not mention who the family members rejecting the marriage to his cousin are, nor the reason for their rejection. If the rejection is from someone other than the parents and is unjustified, then he may marry her without their consent, provided she is religious and of good character. However, if the rejection is for a legitimate Sharia (definition) reason, then their consent is obligatory. If the rejection is from one of the parents, it is advisable to reconsider the marriage, as obeying them is obligatory. But if his heart is attached to the girl, and she is religious and of good character, and he fears harm from leaving her, or if the parents' rejection is due to obstinacy, then he should try to convince them. If they do not agree, he may marry her and then seek to appease them.