In practice, if they confirmed the wallet address they can see if the money is ever moved. If it is, they can charge the guy again.
And if he sells the whole wallet in the future, the buyer is likely to touch it, proving known access.
Sounds like the only way around it is to wait for the statue of limitations.
I am not sure how the law stands here, especially in Germany, but if using stolen assets is or is made a crime, waiting out a statute of limitations on the original crime may not be useful to him.
You can never be 100% certain but cold stored only Bitcoin or seizing all his computers would probably do the trick. Maybe he has the wallet stashed somewhere still, but they would know if he accessed them, at least by keeping an eye on those addresses in the block chain.
Sounds like the only way around it is to wait for the statue of limitations.