The son has every right to set boundaries, especially considering the painful history. It's unfair for the mother to push a relationship with Tim when the son has clearly stated his discomfort. The mother should respect her son's feelings and choices, rather than prioritizing her own desire for a blended family. Forcing the issue could further damage their already fragile relationship. The son is not obligated to accept someone who was the cause of significant family trauma.
The son is being unreasonably rigid and holding onto resentment. While the past is undeniable, refusing to even meet Tim after years of reconciliation is selfish. The mother has made an effort to rebuild their relationship, and the son's ultimatum is jeopardizing that progress. It's hypocritical to demand acceptance while refusing to extend the same courtesy to Tim. The son should consider his mother's happiness and make an effort to move forward, even if it's uncomfortable.
The son's feelings are understandable, given the circumstances of his parents' divorce. However, his mother has attempted reconciliation and deserves consideration. While he is not obligated to embrace Tim, refusing to even meet him seems unnecessarily harsh. Both parties need to compromise: the son could consider a meeting, and the mother should respect his boundaries if he remains uncomfortable. Continuing the ultimatum only perpetuates the conflict.