That speaks to latent speed you might want to investigate, as I mentioned earlier. Totally different energy systems which XC doesn’t help with, but I’m not his Dad or Coach. Coach may have different reasons for keeping him in middle distances. Is he the fastest quarter-miler?
On the nagging thing, being a parent trying to make suggestions to a track kid with potential in their best interests is it’s own level of hell. Don’t envy you. Where I’m at now is it’s gotta come from the coach, who they’ll listen to much more than you. Good luck.