If someone has PKU this means they received two PKU genes, one from each parent. If your son does have PKU it will mean that you and your wife are both carriers. This does not mean that you or your wife have PKU. It just means that you both have one PKU gene and one normal gene, and you both passed on your PKU gene to your son. Please keep in mind that in no way does this mean any of this is your fault as there is no way you could have known you carried the PKU gene.
Like myself Nicole was diagnosed with PKU through what is called the Newborn Screening. This is a blood test that looks for several genetic disorders including PKU. If this blood test shows high levels of phenylalanine (an amino acid found in protein) it could indicate that the infant has PKU and therefore carries two PKU genes.
I hope this makes sense, and if you have anymore questions please let us know!