I doubt they will be checking everyone's birth certificates to verify age. If he is tall enough, and if you feel he can handle the slide, and the splash into the water at the bottom (some children get overwhelmed by a splash in the face), then I'd say give it a try. Worst they can do is ask you his age if they think he looks too small. But I'm betting, at his height, they won't even ask.
Now I'm not a big rule breaker, but I think its mostly a guideline in this case, and that the height is the important factor.