You can ask for him to pay child support based on the 7500 for the time between when you filed your request for modification and the time that he lost the job. Then after he lost the job, the support to be based on the unemployment benefits.
In regards to a modification for back support before a court order, you will not be able to modify it her date of birth.
This answer is a general interpretation of the law and is not fact specific to your case. Likewise it does not create an attorney-client relationship. You should seek an attorney for a review of your specific facts and documents. My firm is All for the Family Legal Clinic, Inc. a nonprofit public benefit corporation that charges on a sliding scale based on income and family size. For a consultation contact us 510-999-7732 or at our website www.allforthefamilylegalclinic.org
The prior attorney is correct as to what you can ask for. You will have to be able to demonstrate to the court the $7500/month salary for the period of time that he was making it if he hasn't submitted any supporting documents to the court.
The earliest you can request child support to start is the date of filing of the Petition in your case. When you file your first motion for support, you have to ask for retroactive support to the date of filing of the Petition, or it will likely be considered waived. If you want support from birth, you should start the process before you actually give birth.
The answer above does not constitute legal advise and is not based on any confidential information provided by the poster. Each situation is specific in nature and any answer offered is based only upon the information provided by the poster; the attorney does not warrant the answer is applicable to the poster's situation. This answer does not create an attorney-client relationship.