If he is certain that urine is exiting from him during ablution, then his ablution is invalidated. He must remove the impurity from his clothes and body and repeat the ablution.

However, if what he feels regarding urine exiting is merely doubt or illusion, then ablution is not invalidated by doubt and illusion. He should not pay attention to it, and it is not permissible for him to follow the doubt and whispers that corrupt his religion. And Allah the almighty knows best.