How do i stop my sons annoying habit of pulling at his cowlick?

If this is his worst: ...Bad habit, leave him alone. Breaking a benign habit usually leads to acquisition of another one, possibly worse.

Difficult: Alot depends on age. Either way some of this is habit and some can be related to other disorders. Either way the best answer is to use positive reinforcement. It won't stop by nagging. So offer rewards or stars on a chart for not pulling his hair etc.