The skilful, attacking midfielder has enjoyed an outstanding campaign as Brentford push for a play-off place in the Championship.
Alex, 21, has scored 11 goals in 44 appearances for the Bees with another seven assists.
Brentford are one of four clubs who can still take a play-off place alongside Ipswich, Derby and Wolves. They face Wigan at home in the season's finale next weekend.
Alex has now been capped six times by England Under-21s and featured in both matches against Czech Republic and Germany in March.