You should serve interrogatories and document requests.
If the cross-defendant does not repsond, or does not repsond properly, you can make a motion to compel.
I would strongly urge you to consult immediately with a litigator skilled in this area of the law.
PLEASE NOTE THAT THIS RESPONSE SHOULD NOT BE CONSTRUED AS A LEGAL OPINION OR ADVISE, AND DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP. IN ORDER TO RENDER A LEGAL OPINION OR ADVISE, THE RESPONDING ATTORNEY WOULD NEED FAR MORE INFORMATION THAN HAS BEEN PROVIDED, AND WOULD NEED TO BE RETAINED PURSUANT TO A WRITTEN FEE AGREEMENT.
That document is only to be used in a limited civil case, and only just before trial. he cannot introduce any evidence, or non-party witnesses that are not listed in his response. If you sent it to early or too late he does not have to respond.