This is difficult and I understand your desire to want to go home. However it may not be that easy.
To start with, in answer to your questions:
1. Yes, he can stop you from leaving if he gets a prohibited steps order granted.
2. He can seek custody and may get it if the argument is that your child would suffer less upheaval by remaining in the country of his birth.
3. If he gets the above, you would be allowed to visit Australia but not reside there.
4. Contact the child support agency about getting
child maintenance. They will contact your ex about money and assess his income.
5. Request details of his income via a
form E. if you feel he is being untruthful then you can challenge him.
The problem for you is that your son is British and the courts may not be happy for you to remove him to another country. In addition is the fact that both parents must give their consent for a child to be taken abroad.
Financially, you are in a much stronger position. You will be entitled to at least 50% of the marital assets, and maybe more if you are the main carer. It does not matter how much or little you contributed to the business, in law it is a joint asset.