I honestly don't see what the big deal is with the word 'lad,' lol. And Coen won't either. Of course age doesn't matter with Athanar's sons, they're lordlings (also the one is a bit older anyway)! But Leof is a stablehand, and quite a bit younger than Coen...thus lad.
(He is around 16, right? I know that's not as young for a man in Rohan as in 'modern RL,' but still, enough younger than Coen to be a lad!)
Quin's a little older than Leof, so maybe he's not a lad. But maybe he is...again, just because of the age difference. They're so young in Coen's eyes!