The 20-year-old recently returned from a spell with Sheffield Wednesday in which he made just four appearances and now drops to League Two with Paolo Di Canio's side
The 20-year-old recently returned from an unsuccessful spell with League One's Sheffield Wednesday, in which he made just four appearances.
However, Paolo Di Canio's promotion-chasing side have sought the highly-rated youngster's services as they look to consolidate their position at the top of the league with nine games remaining.
Lambeth-born Bostock has yet to make a Premier League appearance for Harry Redknapp's team but has nine England Under-19 caps.
He became the north London side's youngest-ever player when he made his senior debut in a 2008 Uefa Cup clash with Dinamo Zagreb.
Bostock's first appearance for the Robins could be at Wembley, as the Wiltshire outfit face Chesterfield in the Johnstone's Paint Trophy final on Sunday.