|If he is a long way from being promotable, then the fact that you have no supervisory positions open right now has nothing to do with his question. |
It sounds like he is under the mistaken impression that if he keeps asking to be promoted, sooner or later you will say yes. The reality is that if he truly wants to be promoted, he needs to spend less time asking about promotions, and more time performing effectively on the job and upgrading any of the skills that are required for advancement.
Without making any commitment about future promotions, you should provide him with some specific guidance regarding the performance criteria, work behaviors, training, and leadership experiences that are essential for those individuals who would like to be considered for promotion. He also needs to be advised that constantly asking to be promoted is a behavior that works against one's chances of actually being promoted.