Once again, a very good source for respirator masks of this quality is Harbor Freight tools. I’ll admit I was a bit anxious (and claustrophobic) using one the first time, but I’ll swear by it!
One problem with respirators is younger modellers. My son’s 10 years old, physically no slouch (5’1", 120lbs), but we can’t find one that fits him well enough. I ventilate the room we use with small “personal” fans such as , along with the furnace fan going during the session. He also wears one of those ‘surgical masks’. It’s not the best solution, but it permits him to pursue the hobby with a marginal amount of safety (better than no safety at all).
At least you are trying to do something for him. One alternative that you might consider are a spray booth. There are several ideas here for building them inexpensively. Also, and you may have already explored this, is that 3M makes respirators in several sizes. I guess I’m pretty “Average” as far as face size goes, and I use the Large size. The “Small” or “Medium” size might fit him. I got mine at Home Depot. The Large is model 6300, the Medium is 6200, and the Small is 6100.