He is legally required to help support the child, if it is his. He is most likely scaring you with the joint custody, as my experience is that a Judge will not award joint custody to two parents who do not agree on the joint arrangement. In some jurisidctions you may be eligible to get support retroactive to the date of birth, and including some delivery costs. Obvioulsy, he wants the paternity test to insure that it is his, but that is pretty minimal considering how much you have to gain, if it is indeed his child.
Child Support and Child Custody are inter-related in that often the threat of custody is used to discourage seeks support. Your decision to raise your daughter on your own may not be completely fair to her. As she grows older she will be impacted by her father's seemingly disinterest in her. But, seeking support is going to open up what has so far been neglected. I suggest you go through with the paternity test, at least for your daughter's sake, and if he is the father then the test will confirm it. Your daughter will know who her father is, and he can seek partial custody, but in Pennsylvania, one parent will have primary custody of the child. I seriously doubt that the court would change the status quo at this time, unless you try to keep your daughter from seeing her father.
The question is what does he mean by "joint custody?" Is he talking about shared physical custody and threatening that he will seek to have your daughter 50% of the time or is he talking about periods of partial physical custody? Shared physical custody is not likely to result, but it probably just a threat to keep you from seeking support. Partial physical custody means he has her some of the time and unless he is not fit as a parent that is probably a good thing. If he does seek partial custody, it is likely that the Court will start out slow with perhaps a dinner visit for a few months working towards an eventual over night and ultimately perhaps even an alternating weekend type schedule. If she has no other father figure in her life and if he is a healthy father figure when he enters her life, this is probably in her best interest. However, if he is not then you can weigh the pros and cons regarding the support you need for your daughter and the ramifications that having him in her life might present.