S.H.
Bike shorts/pants, of which they come long too. It is warm.
Or, Thermals
By 'leggings', the Coach means, bike shorts/pants or thermal type things.
Just Google Search "athletic leggings for boys." Or "bike pants for boys."
Then these won't look girly.
It is for athletes.
Or try "athletic leg warmers for boys." Google search. There is such a thing. Athletes wear them.