How can I get the correct amount of child support if my ex didn't provide his financial affidavit to DOR?

You can either: (1) convince the DOR and their lawyers to re-open the case, or (2) hire your own lawyer. Either way, his income records (tax return, W2, pay stubs, bank records) will need to be subpoenaed to support your claim that his income has increased.

If it just occurred, a motion for rehearing needs to be filed asap - it must be done within ten days. The statute provides that income can be imputed above the minimum wage. You can also retain private counsel to assist you with this.

I agree with both attorney Mullaney and Augen. There are tools at the attorney's disposal to actually find fathers true income, such as request to produce, interrogatories, subpoenas to employers and financial institutions, etc.
The time and effort required to do this may be substantial and unreasonable for DOR to make in every case. If you cannot convince your DOR attorney to go down this path, your only other option is to represent yourself or hire an attorney to file the appropriate case in circuit court.