The Court of Domestic Relations has continuing jurisdiction over the issue of child support. Your brother can request a modification of the order for the child who is no longer living with the mother. That is a change in circumstances that would "hold water". No one, however, forced your brother to follow the advice of his previous legal counsel; I doubt he would prevail on any claim in that regard. Your brother must have agreed with his advice at the time. The "new evidence" as to paternity certainly would probably not fly. To undue the prior order he would need to file what is called a 60(b) motion and it sounds like the time has passed for that. The paternity evidence, is not a change in circumstances, however, and unless the "real" father steps up, I doubt anything can be done. The court will not want to leave the kid without financial support.
Yes, there is provisions under Ohio law to disestablish paternity under some circumstances. It is best that your brother consult with an experienced family law attorney to discuss the possibility of determining if his situation would fall under the provisions of law. If the attorney believes that there is potential to have the paternity disestablished to cancel the child support support (no money is refunded, just future support is terminated) then he should discuss going ahead with the testing.