Thomas was previously on loan at St. Andrew's for three-and-a-half months at the end of the 2012/13 campaign.
During that time in the Second City, he made a total of 11 appearances (including five starts) and scored three goals.
Ahead of arriving at Blues for a second time, he was on a short-term contract at Rotherham United, and was part of the side that won promotion to the Sky Bet Championship via the play-offs at the end of the 2013/14 season.
The Londoner initially cut his teeth in the non-league game with Waltham Forest, Thurrock and Fisher Athletic before joining Dagenham & Redbridge. Following loan spells at Grays Athletic and Rushden & Diamonds, the striker joined Cheltenham Town before moving to Crawley Town.
After a short spell with the West-Sussex based outfit he joined Bournemouth. Following a season with the Cherries, he went out on loan again, to Portsmouth, Blackpool and Blues. He then made the switch to Rotherham in January 2014, where he found the back of the net six times in 16 appearances for the Millers.