Is a young man's second marriage, despite his parents' objection which reached the point of pressure and attempts to separate him, considered disobedience to his parents, and is he sinful for it?

It is not permissible to disobey one's parents in matters that cause them harm. Therefore, a son must obey his parents if they forbid him from marrying, unless marriage is obligatory for him because he fears falling into zina (unlawful sexual intercourse). In such a case, obedience to Allah takes precedence over obedience to them. Marrying a second wife is subject to the same ruling as marrying the first.