If you really think that he is getting a separate check for overtime, I think you definitely need to go to Child Services. One other thing, if he has been in the same job all along, his pay would not decrease; thus, you do not have to be afraid of getting any less.
The question becomes, how will you try to collect the money from him once you found out he is not claiming the right amount of income? If you don't go to Child Services, you will need to go to an attorney to petition to the court.
If the situation is correct and the company is doing this to help him, it's unethical and should be reported. However, I am not sure if his employer is will to go that far for him. I can see this happening if the company he works for is very small and he is intimate/close with the employer.
I have been an auditor for over 8 years and I have not seen a copy that pays a separate check for overtime because it naturally cost extra money for the company to process extra paychecks. Plus, the company has to track YTD information in order to pay unemployment and social security tax. With all these requirements, there is a lot that the company is going through for him which they can get into trouble for.
I am so sorry you have to go through this. I hope that you can make it work.