We know that a Boggart exists even if nobody sees it:
Nobody knows what a Boggart looks like if nobody is there to see it, although it continues to exist, usually giving evidence of its presence by rattling, shaking or scratching the object in which it is hiding.
Now, I can, for example, imagine a situation of an older brother telling horrible stories from Hogwarts to his little brother to scare him.
It could also include a story about this really scary class where the Boggart is taught. This creature which transforms to your biggest fears.
The story would make the little one so scared of a boggart that it would become his biggest fear despite the fact that he didn't ever saw it.
Would his boggart have its natural form?
I don't need to know how the natural form of a boggart looks, but if this is an in-universe way to find it out.