He just analyzes how I did this and after some time secretly re-implements it from scratch with very minor changes or additions.
The answer here has to be Code review.
Incorporate it into your workflow, then there's no "secret code" being committed. (The fact someone can commit code in semi-secret is kind of an issue in itself.)
He then has to work in a branch and have his re-implementation signed off by at least one other developer before pushing to master. If he tries to get weird re-implementations for no clear reason signed off with minor changes, then that will immediately raise red flags to any half-competent reviewer.