This is one of those parts where they've kind of messed with the timeline for the sake of TV. In the book, Laenor is younger than Laena, and even if he were old enough to ride a dragon into war, the book doesn't mention him as playing a role in the War for the Stepstones.
(future plot spoilers ahead)
[spoil]Rhaenyra does end up marrying him, though he's flamingly gay, so only out of duty. Their kids look nothing like him, fueling rumors that their father is really Ser Harwin Strong, the son of
the guy who actually gave the King good advice in this last episode.