Technically since goldfish carry eggs and they don't get fertilized until they have been layed, goldfish cannot "be" pregnant. But they can become "egg bound" or gravid. Two other possibilities are constipation or some sort of tumor. If it is constipation then I will recommend what was told to me when I had a constipated fish. Stop feeding for several days and see if she eventually deficates and the swelling should go down. Fish can develop tumors like any other creature and if this is the case then I would simply let the fish keep living unless she starts showing signs of pain or distress. Look at her symptoms to get a better idea of what is going on. Is she still eating and swimming normal? Has she stopped eating and just sitting at the bottom of the tank? Is she swimming sideways or on her back? What do you feed her, and how often? Regardless, there are ways to humanely euthenise her when the time comes, but please do not flush her down the toilet if she is still alive. The chlorine in the water would burn her skin the way acid would burn yours, and it would be a painful way for her to go. www.wetwebmedia and www.aquariumlife.net are two good websights that can help you become more educated about fishkeeping. Good luck to your sons's goldie.
What kind of goldfish is it? Ryukin goldfish have naturally rounded bellies and this is normal for them. If your fish seems abnormally swollen it could be several possibilities. Constipation, egg burden or even a tumor. You need to provide more information to get a better answer. Try www.wetwebmedia for further information. Good luck to your goldie.