You need to ask for the Detective Sgt. and explain how nothing is being done. Be polite and ask for suggestions. If nothing happens file a complaint on the basis of failure to act. The police are busy, but it should not entail dropping the call.
The other avenue is to file a motion to produce with your local courts and have mom served at work to divulge the contact info for Jr. If she fails to comply with the court order, she can be held in contempt. Once you have his info I would file a suit against him. It's much quicker than the criminal process.
Just my 0.02
