From what I've seen this season and comparing it to the 'he's going to be ace' forecasts we have heard that many times already with many other previous prospects, and also then with the increase he is now demanding, then I would let him go.
His corners and crossing in some of the Europa League games were poor to say the least and his positioning was suspect a few times. I know he's young but there's too much what ifs with this guy and even if he does stay, whats the chances he won't start stamping his feet again in a year or two!
We don't need players who aren't even first team regulars (especially out of contract ones) demanding this that and the other after the season we have had.