How tall, how old, how thin? Photo would help.

Rowans are supposed to be 'leggy' in a way, after all they are trees (except for a few rarer bush types).

Is it in a spot where a tall tree would be unsuitable? If so might be better to remove it before it gets too big. Trying to keep a potentially big tree small usually ruins the shape.

they can be pollarded quite successfully and look reasonable. the council did a row of them in a street I used to live in.