umm, well 6.7 for a 14 year old isn't rare so i doubt he's lying. did he develop early?
If you want him to show you, use the old porn trick. While you two are on your computer, just search for some porn or something and if he looks turned on, start jacking off and see what his reaction is. 50% he'll get his cock out and join in, 50% he'll think your a freak and go home but us brits are pretty horny and up for most stuff.
It's up to you though, anyone else with ideas?