If the hair growing is the normal hair that people and grownups shave, you have to cover from him as he is an adult now.
Even if he didn’t grow this hair yet, 12 years is a grownup boy. You can see that his looks are not that innocent as this is the time when they start to investigate. That is why, you should cover from him, as it is also good to teach him modesty and to teach him to begin to separate from women’s gatherings.