Funny, I listened to the thing a couple of time and I was always hearing "pojken li trött" with the fast audio, and "pojken vil trött" with the turtle audio, which did not make any sense. But now that I know it is supposed be "blir", I can hear it properly with both (I don't hear the "r", of course, but that's how swedish works I guess).
French is my first language. What about you all?
I agree with you that "The boy is tiring" is absolutely fine, and has the same meaning. We've discussed it internally as well and reached the same conclusion - that how you use "tiring" depends on where you're from.
However, we generally don't allow constructions that change POS, so since it moves from an adjective to a verb, it still won't be accepted. It's a guideline we try to enforce throughout the course, to make it easier to stick to learning the Swedish words better.