Hi Ben,
been in your situation still in it fighting for permanent custody.
There were drugs and alcohol involved so I managed to get a lot of evidence and got an urgent application approved by the high court for urgent temp custody, I still have it 7 months later and still busy with the custody case.
You can start by going to the office of the family advocate for advice and take it from there.
Keep your side clean, be good, dont drink, smoke, drug or anything. Be a saint from this day forward if you are not one already. Communicate with her by email only, keep all records. Be nice, be the best person at all times, no matter what they say or do to you.
Be patient, be strong. If you are not close to God, this would be a good time to start. Read Psalms, it will help you. I could not have got where I am in this matter without my Faith. You have nothing to lose.
Start getting your home ready for your son if it isnt already (room and all that), start paving the way for this to happen and it will. if you dont see him often, see him as often as you can without fail, keep all transactions you do at shops for him. I hope you are paying her maintenance, do this via the internet and NEVER stop doing this! File these transactions.
Be the better person at all times, dont lose your cool. She is on the way downhill, let it happen and be ready to act when things explode, and they will. Start gathering evidence. Its not an overnight process, it can take YEARS but you have to start somewhere.
Find God and take the rest of my advice. be strong, I know how hard and sore it is to feel the way you do, use this all to make you stronger. It can be hard to stay positive but keep on keeping on. Get to the office of the family advocate ASAP, take charge.
You are not alone, may God bless you for the battle to come. Make sure you are ready my brother. YOU WILL SUCCEED!!