Excellent, and I agree with all. Of course, it's up to your son, but it might be a useful exercise to let him brush-paint his first model. In this way, he will be able to understand the uses and limitations of paint, how it behaves, how to apply it in the best way for the job, and the need for proper and thorough stirring before use.
Using an airbrush for the very first time can be a daunting task, even for an adult, and mistakes, big errors or problems, could lead to disenchantment and discouragement. However, if he insists he wants to air brush his first model, then let him do it - but yes, some form of mask, if using enamels in particular, is advised.
I have never used a mask, but then I grew up in a world where the average airborne poloution would never even be dreamed about today, so, to a small extent, I'm 'conditioned'.