How can one convince a friend who believes that the noble verse, "And there will circulate among them [eternal] youths" (ويطوف عليهم ولدان مخلدون), means that sexual enjoyment of youths is permissible in Paradise, to abandon this understanding and repent from it?

The books of Sunnah and Fiqh are replete with condemnations of sodomy, considering it among the vilest of indecencies. So how could it possibly exist in Paradise? Your friend has misunderstood the verse and will not find any interpretation that supports his assertion. The beauty of these youth is to further honor the inhabitants of Paradise, and their sole function is service. Indeed, a king loves his servants to be in the best possible form. However, to think of sexually enjoying them is an abomination, so how could it be for the people of Paradise?