Breanna is right – at this point breastmilk is the best thing your wife can be giving your child if he actually does have PKU, because regular baby formulas have way more protein in them then breastmilk. I have breastfed two out of my three children with PKU and each of them I breastfed for 9 months (with the second one getting pumped breastmilk in small quantities for a few months longer). With the two that I did breastfeed I also had to provide a low protein formula at a few of their feedings. With Erica I breastfed her at every feeding and then had to give her an ounce of formula three times per day. With Genavieve I gave her an ounce of formula at every feeding and then breastfed her after that.
As for your second question about your child growing properly – the special formula that he would drink if he has PKU has a lot of nutrients and that alone will give him things such as modified protein to help with growth and development. In my family you cannot tell by looking at my kids who has PKU and who doesn't. In fact two of my more solid kids (I say solid because they aren't fat or even chunky but just feel more solid then their siblings) have PKU. Just like the regular population has various sized people so does the PKU population – there are short people with PKU, tall people, skinny people and even heavy people.