My ex is claiming our son on her disability so she can receive more money and she told them that she has him 51% of the time. Now we have joint custody 50/50 but he's with me about 65-75% of the time.what step can I do to stop that from happening?

You could report it here:
https://www.disability.gov/home/i_want_to/disability_benefits/I_Want_to_Report_Disability_Benefits_Fraud

You should keep record of the times the child is with you. This will also be important for tax issues in case she is trying to claim your son on her tax return.