Is zakat obligatory on a sum of 1500 Saudi Riyals that he receives monthly from the rent of a villa he owns?

If the student only possesses the mentioned amount and continuously spends it such that no nisab or more remains at the end of the hawl (one lunar year), then zakat is not obligatory for him due to the absence of the nisab condition. However, if he saves a nisab or more and a hawl passes over it, then zakat becomes obligatory for him.