Can my girlfriends parents claim our child if I say no but we all live together.

My girlfriend, son and I live together with her parents. She is 17, I am 20 and we lived separate until she became pregnant, at that time we all made a mutual agreement to get an apartment together, my name is on lease as well as theirs and I work and pay my part of the bills for the household so it is not that I live with them, it is that we all live together. Once the baby was born they got the social security card and birth certificate from the mail and will not allow me to have them because they intend to claim him on their taxes. My question is can they? I have a job and I pay my taxes and file return and my name is on the birth certificate as the father but because their daughter is under the age of 18 and we all live under one roof they are going to claim him and say that I can't. We are not married, my job supports the baby, I pay my bills, shouldn't I be able to claim my child? What would happen if we both claimed him? Can i report them for this? An attorney told us that the only way her parents would not be able to claim him would be if my girlfriend told her parents they can't do it but she is afraid of her mom and won't do this. Any answers or details would be greatly appreciated.