To answer your first question if your son does have PKU and is put on the low protein diet after diagnosis he will be okay. He will need to drink the special PKU formula, but if your wife decides to continue breastfeeding as well she can do that. Otherwise he will still need to have some regular baby formula, because despite having PKU your little guy still needs just a little bit of protein to survive.
The PKU formula will provide him with all of the nutrients (protein without phe in it, calories, etc) he needs that he would not get from other foods that are too high in protein, so the low protein diet should not affect his growth. If he has PKU as long as he takes his PKU formula he will grow and develop just like any other little boy.