Depending on how much the project is worth to you, you could
contact a local lawyer, show him your contract with the sub-worker
and ask him to send a letter to him, where you demand him to hand out the
database and the other code you've changed.

This will cost you something arround 300€, but if the contract is obvious,
a letter by a laywer has a certain impact on people, and if he don't
really want to go to the curt with you (as this will also cost him a bunch of money),
he should hand over all availiable data.