This is two seperate issues. The support would only be able to be terminated by the Mother filing a a request a Domestic Relations to withdrawl the child support and any back arrears. She would have to agree to do that on her part. As for the termination, the Mother would also have to agree to do that a well by asking you to terminate your rights as the biological father so that her current husband could adopt her son. But that is something she and her husband would want to do on their part as this is a huge step and undertaking for the step-father. Plus, this would mean you would be ceasing all ties to your son by terminating your rights as his father. This is a lot to think about and consider.
The two separate issues you raise need to be dealt with in different manners, For the child support, you can seek a modification based on your limited income. Pennsylvania offers relief while still requiring responsibility. You may benefit by speaking with a family law attorney. As to terminating your rights, Pennsylvania law does not favor making orphans. If the mother's current husband wants to adopt your son, then you can consent to termination. However your relationship will be surrendered - this is a drastic move that requires your own internal evaluation.
Please know that the information I provide is general in nature as we do not have an attorney-client relationship, and thus do not have all the background information which would normally help me give a specific answer. However, if this answer has been helpful to you, please consider rating it with a "thumbs up". I wish you the best outcome with your situation!