He should resign. I agree that people can change their opinions over time and it is quite possible that he genuinely has changed his own after 15 years.
However, an MP needs to should set an example. I don't think he should be automatically barred from being an MP over these comments, but he should have disclosed them before standing for the role so that the voters could have judged for themselves whether they accept his argument that he has changed his mind now.
He didn't, he kept quiet and hoped nobody noticed, showing himself to be as much of a coward today as he was when he made those comments.