OK, yes your DH has parental rights, but you have misunderstood your reading. If your baby is so tiny and breast-fed, your DH absolutely cannot just run away with him. If the baby was older, your DH might be able to do this, but not with your son the age he is.
I am sorry that your parents are being so heartless. They would rather allow their daughter and grandson to be abused, than to have their comfy lives disturbed by telling an abusive man to leave their home. Disgraceful behaviour.
Put the baby's passport in your bag, or in the safest place in the house that you can think of. I have known people to put passports inside a sealed plastic bag, and duct-tape them inside the lid of the toilet cistern.
Honestly, if you have some money, your best bet would be to rent a safe flat for you and the baby, and get a solicitor. Your parents wont step up to help you (is this a cultural thing by any chance?), so you need to be strong for yourself and your baby. If you are anywhere near Milton Keynes /Luton / Bedford, i will come and help you.