The backlight of an LCD generally cannot be controlled through the digital interface.

You will need a FET (I use an FDC610) with the source connected to GND, the drain connected to the backlight (-) terminal, and the backlight (+) terminal connected to 5V. Your LCD may require a 5 ohm .5W resistor between the (+) terminal and 5V to limit the backlight current (check the datasheet).

Connect a 10K resistor between the FET gate and GND and also connect
the gate to a PIC output pin. Whenever you want to turn on the backlight, bring the output pin high.