Can I control speed of reduction motor?
As its a small motor, you could control its speed with a "Rheostat" essentially a higher wattage potentiometer, 

However the proper way to do it would be with a PWM controller, it switches on the supply voltage X% of the time, proportional to some analog input, say a potentiometer, this keeps the motors speed more consistent over varying loads.

Edit:
Tinkercad exploration image attached.