ANSWER:

The sunnah is that the follower doesn’t move until the imam has reached the final stage of his movement.

This means that if he is going for sujood, the follower doesn’t begin to bend his back until the imam’s forehead has touched the ground.

This is cascaded in all other movements. 

The imam of my masjid, during the course of ruku’,while going down,stops in the middle of the journey for a second maybe because of a back issue,then continues going down and he does this almost everytime.In prayer,i definitely START ruku after him but because he pauses in the middle and i dont, it is possible that i reach final ruku position before him.If this happens, would my prayer be valid?