The 27-year-old central midfielder signed a two-year contract at the club in July 2015.
Simon had spent the previous season with Stevenage, scoring five goals in 33 appearances as the club reached the League 2 play-offs. He made 83 appearances for
Hartlepool before joining Stevenage.
Simon started his career at Leeds United and joined Charlton in a £500,000 deal in 2004. QPR then paid £200,000 for his services and in 2008 Simon headed to Plymouth, for whom he played 66 times and scored nine goals.
Simon has also played for Ipswich, Hull, Blackpool, Crewe and Sheffield United on loan in an 11-year career that so far has included 294 appearances.