How can I report a person who owes child support?

I dont have his social sec. but i have his work info and address. It pisses me off that he has a job full time + he has welfare. He doesnt pay rent or anything else but he wont send his daughters anything. I know that his child sup record was opened in boston. He already owes over $5,000. its not really my business but he is my brother in law and his daughters are very special to me. I lost contact with them and the mom 4 months ago so i cant call them directly. I was also a child sup receiver when i was younger and i never recieved anything so i know how the mom feels. Any help will be appreciated. Oh and dont call me nosy if im looking out for 2 little girls well being thanx in advance