I very much doubt you could take the baby without the parents consent, it would probably be considered kidnapping and he would be returned to his mum. His mother is an adult and has to make her own choices, obviously hard for you to watch if the baby is at risk.

I can think of two things you could do, one would be to contact social services, as this man is known to them they would more than likely investigate the situation, the second would be to apply for a residency order. However I would go for the first as any threat to take the baby could alienate her from you altogether and also because social services are more likely to provide her with long term solutions that will allow her to move on from her need to be with this kind of man...and keeping mum and baby together in a safe enviroment has got to be the ideal solution.