It's too much instability for this little 5 year old boy. He has a biological father, an abusive live-in mother's boyfriend, a difficult grandmother, and now another mother's boyfriend. The girl needs to establish independence and provide a stable, loving home for her son. Please don't contribute to this ongoing problem. The problem does not lie with rescuing the girl or her son. The problem lies with the fact that the girl is not mature enough to realize that she needs to be a mother, without a boyfriend right now. Her entire focus should be on helping this little boy develop trust, stability, and love so he doesn't grow up to be an abusive boyfriend, a father who abandons his family, or a freeloader.
Your 27 year old son also needs a place of his own. If he chooses to move this girl in with him, that's his choice, as he's an adult.
I have two children. One is 27, one is 23. The 27 year old, my son, lives on his own and works. He has a college degree. I want to help him, because he's my boy and I love him. So I help him. If I'm cooking something I know he enjoys, I make a serving for him to take to work for his lunch (a couple of times a month at most) and he comes by and picks it up, and thanks me. I meet him once a month at a local cafe to catch up on his job and life, and visit, etc. That's the kind of help that is actually helpful. It gives him some good food and a break on his grocery bill, but he knows he can't eat here every night. You're not helping your son by allowing him to live at home since he seems to be a healthy normal young man with a job.
My 23 year old daughter lives at home, because she has serious medical problems, and cannot drive. She'll probably live at home for a long time. I drive her to doctors' appointments and manage her medications and insurance paperwork and help her by cooking her food, and basically taking care of her. That's the kind of help she needs. It's a requirement, and my husband completely agrees with her living with us and although I do most of the care, my husband works hard so that I can stay home with her.
Please stop helping by enabling dependence on you. Tell your son he has 60 or 90 days, or until January 1, 2016, to get his own place. But in the meantime, no roommates. The girl may need rescuing, but not by a casual boyfriend's helpful mother. Her focus must be on her son, and no one else right now. Moving her in to your home would allow her to escape all her responsibilities, even if she helps with the vacuuming and dishwashing. There's too much risk for this little 5 year old boy.