It makes no sense to do that. Either way he'd be playing for Saints and gaining value. By selling him and loaning him back he is gaining value for Chelsea and not Saints. So why not keep him, not as if we need the money with the Liebherr trust backing Cortese's ambitions.
He will be signing a long term deal for Southampton Football Club on his 18th birthday (12th July 2013) like James Ward-Prowse did with a 5 year deal...