First of all congratulations on your new little boy! I cannot imagine the emotions your family must be going through, but please know that with proper treatment your little boy will lead a very happy, healthy, and productive life. Treatment for PKU has come so far since it was first discovered, and I am sure it will continue to move forward.
On a different note please do not think that this diagnosis was anyones 'fault'. In order for your son to be born ith PKU that means that he had to receive two PKU genes. Each parent can only pass on one gene, which means you passed on one PKU gene and your fiancee passed on a PKU gene. You and your fiancee are carriers, which means you each have one PKU gene and one non PKU gene. You obviously do not have PKU, you are simply carrying the gene. Your brother and sister could also be carriers, but the only way they would have children with PKU as well would be if they married someone else who was a carrier and could pass on a PKU gene as well.
Take care, and use this website. Having a support system in place will help a lot, and I am sure everyone here would be more than happy to help you along the way.