Add a new feature that uses new data, and encrypt that data. Not an insurmountable obstacle, but it will put you ahead of him in the comparisons on the app store as long as you can stay ahead.

But he can do the same thing to you.

In the end, the users win. That's the beauty of a platform that has critical mass in numbers of developers competing to make good apps. Sucks for you (us) unless you look at it as a fun challenge.