Casey, 18, signed for Boro as a professional in May 2015 but was released at the end of last season without making a senior appearance.
In January 2016, Casey joined Evo-Stik League Southern Premier Division side St Neots Town on a month`s loan.
He had made sixteen appearances during a six-month loan spell with the Saints` league rivals Chesham United earlier that season.
His performances for the Saints led to them extending his loan period until the end of the 2015/16 season.