If he acted like the Dad then a later test may not get him out of the acknowledgment.
See a PA attorney on this right away. This is too complex for an online forum like Avvo
It really depends on the specific facts. The common law presumption is that a child born in a marriage is the child of the husband, yet with the advance of DNA testing some of these presumptions have begun to be turned on their heads.
Is this in a marriage?
Has the man acknowledged the child or has his name been put on the birth certificate by the mom?
Sometimes this is done unilaterally without the knowledge of the man, and, if so, and he can establish that he isn't the Dad through DNA testing, then he probably will not be required to pay support.
The man should have an attorney to represent him.
I am licensed in New Mexico and Pennsylvania, and therefore any discussion of issues related to other states must considered within that context. In addition, my comments are not intended to create a legal representation but merely to respond to the limited facts presented by the question. Any opinion herein is not meant as a precise statement of legal rights or as a recommendation of any particular course of action. A more complete legal review can be obtained through local counsel.
From the facts you have presented this gentleman may be estopped from challenging the issue of paternity at this time and will have to continue paying support. Typically if one signs an acknowledgement of paternity, DNA tests are not even offered. Your facts indicate that he was offered testing and rejected it. Make an appointment with an attorney in your county that handles support matters on a regular basis.