Hi OP, I have worked in domestic violence support services as well as having experienced it myself as a child and an adult. Others on this thread have been fantastic, and I just wanted to post to offer the following advice -
You are absolutely in no way at fault, and the guilt/confusion you may be feeling has been programmed into you by his behaviour. It is cunning, manipulative and designed to make you stay even when things are truly terrible. Please remember that this person is not capable of loving you in a healthy way as you deserve. You are only of interest to them because they wish to control you, and I know that's hard but it is true.
You've done some really impressive ground work to prepare yourself for leaving, such as ensuring anything important is already at your parents home. Please try to ensure you have taken all essential documents for yourself and your son out of the home before you need to go, but only if you can do this safely without it being detected. The same goes for any sentimental items too.
Remember that technology is very impressive now and that we can be tracked in ways we may not consider. I would strongly advise turning off any ICloud (or similar) settings on your phone as soon as you have left, and if possible it's even better to have the phone completely replaced. At the very least, go into somewhere like carphone warehouse and ask that they check the phone for anything unusual like a tracking device. You don't have to fully explain yourself to them but they will have seen this before and can look for you (some devices are tiny and can be hidden well) This is also important for any social media. I would personally suggest you don't have any accounts active as it makes it too easy to trace you.
If you drive, get your car checked for tracking devices too. It may seem extreme, but where I worked it was a common theme that service users found out a device had been fitted to their car. Get a garage where you live at the moment to check for this before you go to your new area so that he doesn't have any knowledge of your movements. Again, mechanics will have seen this before (unfortunately!)
If you are able to have a clear leaving plan, try to make it so that your son isn't there and you have someone with you. I see that you mentioned your brother helping, and that is great. The most dangerous time for anyone experiencing DV is when you are leaving the relationship, so be as quick as possible and be with someone you trust to help you. It's also a good idea to have someone totally aware of what is happening who isn't present but knows when to expect you or is waiting for phone call to confirm you are ok, with an accurate timescale of when to call the police if they haven't heard from you.
Police have specially designated teams to deal with domestic abuse and of course it is your choice of whether you wish to use them, but they are trained to help you. They have specialist officers who can go through a questionnaire with you to deduce your level of risk and support you appropriately. This can often be a real eye opener to the abuse but also can really help in ensuring your safety. If they are aware of your leaving plan, they can put safeguards in place, and they can also transfer the support to your new area for a wraparound effect. They can also signpost you to excellent support services once you have left who will help you to understand what you have been through and keep you feeling strong. There are also specialist services for children who have experienced domestic abuse which can really help them to understand and move forward.
Do everything you can to stay in the mindset and not contact him. He will throw everything he has at getting you back, so it's best to be completely unreachable and ignore anything that you hear. He has done this before, he will do it again, and you deserve so much better.
If you are able to speak with the police, they can also ensure a priority flag is placed on your current and new properties which comes up whenever a call is made from those addresses so that if, for example, you call but can't speak, they know the nature of the issue and can respond with an appropriate urgency and knowledge. They can also have a safety specialist come to your new home to look at locks, inform you of any weaknesses in home security and fit cameras for you free of charge. They can also fit panic buttons if needed.
Make your sons new school aware of what has happened. I know that this part isn't easy, but you have a huge advantage in that he isn't your sons father and has no PR. If they are aware and have a picture of your soon-to-be ex they can be totally prepared to keep your son safe and will also be able to understand and support if he needs it.
You will one day soon be able to look back and be proud of this, and it will feel amazing. You have a great future ahead of you and are being the best mum you can be by getting out for you and you sons sake.
If you need any more advice please let me know. 