Tough times OP, I'm sorry you are going through this. Like (unfortunately) many other women, you will one day look back with awe at how you got through this next phase of your life. It will involve tears, hate, despair, and then one day you will surprise yourself by laughing at something. Tears follow again and anger may follow again but that's a normal part of the process.
It is important to acknowledge to yourself that although it feels shit, it is absolutely the right thing to have done. I found thinking about my future in three phases helpful...short, medium and long term. Housing, finances, childcare, work etc. The long term was too hard to sort so i just focused on short term - today, or in a stronger moment this week- initially and then medium term eg 6 months.
With regards to what you tell your son, my dc was two when we split too. I told my child that "Dad just wasn't able to be a good Dad, and you deserve to have people in your life who are good enough and treat you properly" leading onto to prompting them to talk about who our trusted people are such family and friends, and what treating someone properly looks like. E.g Aunty X always listens when I'm sad and she makes me laugh too. Friend Y is helpful and kind.
That sufficed for a number of years, until they were about 8 or 9, when the questions became more in depth...older siblings have shared snippets and I don't censor them, their truth is important and valid too. The timing of questions can ebb and flow, so they may not ask for 6 months then ask every day for 2 weeks. But I go with the flow and have learned to trust my own judgement after doing a lot of recovery work. So far this has worked and dc is a happy, well balanced child with strong self esteem and shows self worth in their own relationships with friends.
Most importantly the former two yr old is no longer damaged by the abuse that they were being exposed to, and I hope will grow up into an emotionally healthy adult.
You've done well to make the break, look after yourself so you can look after DC, and give yourself credit for the strong stance you have taken to protect yourself and your DC.