If he has had a new baby, that would justify a change in circumstances that would allow him to move for a modification. The good news for you is that his income has increased, and he makes twice as much money as you. Let him take you to Court. If he wants 50/50 custody he will need to convince the mediator and the Court that it is in your child's best interests. Don't let him bully you into receiving less support. I would advise that you go at least consult with a family law attorney who can help you prepare a response to his motion, run support calculations, etc. Good luck.
Please note that this answer does not constitute legal advice, and should not be relied on, as each situation is fact specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship.
I agree with the first response, but I would add this. If the father makes $200,000 and that is twice what you make then you are in a position to pay for a good family law attorney to help you. Depending on the circumstances the attorney may seek an award of all or part of your fees from the father. The sophisticated way you set out your facts and your familiarity with the dissomaster all would indicate that your long term interests would best be served by having professional representation. It wil cost you money up front, but you will almost certainly receive a better result with an attorney from your area, familiar with your family court judges and how they work. Remember that child support, unlike spousal support, is essentially based on the amount of income. Of course the new child will be factored into the complex algorithmic calculation called for by the statute.