What can I do if my wife won't let me see my son?

My wife and I do not live together, and we have not yet filed for divorce. My two daughters are with me, and my three-year-old son is with my wife. My wife and her family refuse to let me raise and care for my son. She hasn't returned my calls in the last three weeks. Her brothers are complicating matters by urging her not to allow me to serve my son's day-to-day needs. I require legal advice and the cost of obtaining guaranteed access to my child from the court, as well as protection from her family members interfering.