Charge him with what exactly?  I'm not sure he broke any laws - his behaviour may be questionable in many people's minds, but its not illegal.  The girl was 16 (legally capable of consent in this country and most countries around the world) when the relationship began.  Intercourse didn't happen until she was almost 18.
There's no reason to assume he used his "position" to any undue advantage.  This is a private matter between those involved, not a tabloid news story for the holier-than-thou crowd to revel in.