First of all congratulations on your new son :). I am sure having a new baby in the house is a very exciting experience, but considering the circumstances it is probably a bit overwhelming as well. Take a step back and just cuddle him! He's going to grow up fast!
With proper treatment your son will be just fine. I was diagnosed and put on the low protein diet at nine days of age. My initial phenylalanine level was much higher than your sons, probably around 1200 ummol/L. Today I am a happy, healthy nineteen year old college student studying nursing. I monitor what I eat, drink my metabolic formula, and do regular blood tests. Other than that I am able to go to college, dance, work in a child care center, and do virtually everything other college students are able to do.
Having mild PKU just means that your son has a little bit more enzyme activity. This means his phenylalanine level is a little bit lower than someone with classical PKU, and he might be able to tolerate a little bit more protein in his diet. Treatment for classical and mild PKU is exactly the same: a low protein diet with a metabolic formula. The low protein diet usually excludes meat, dairy, beans, nuts, and most grains, but there are many special low protein foods that he will be able to enjoy. These foods are low in protein and meant to imitate foods such as meat, breads, pastas etc. The metabolic formula will provide him with any nutrients he is missing in his diet.
I hope this helps, and please don't hesitate to email me at firstname.lastname@example.org or send me a message here if there is anything I can do to help.