H.Evo is a unique.

Cool, thanks.

Can H.Evo turn off his "GROWTH" Special to become normal size?

He certainly can, just as Emperor Joker can do the same (unless they errata him to say he cannot turn off the power, like they did with the Logo figure as I recall)

No errata has been issued regarding EJ, so he can indeed turn off his Embiggen power when he wants to. As the question was asked fairly early on, it probably would have made it into the errata by now if the intent was that it should not be optional.