First of all congratulations to you and your wife on your new baby boy!
It sounds like your medical team is using milligrams per deciliters to measure Dominic's phe levels. When phe levels are being measured in mg/dl they should typically be between 2 and 6 mg/dl. I am not positive, but I think my initial phe level before starting the low protein diet and formula was around 20 mg/dl, so much higher than your sons. Your medical team should be able to bring his phe levels down pretty quickly by restricting how much breastmilk or baby formula he is drinking and replacing it with a little bit of the special metabolic formula. In the beginning it will take a little bit of trial and error to determine how much phe his body can 'tolerate' (ie determine how much phe he can eat while still keeping his blood levels within safe range).
Your son shouldn't face any ill effects so long as he drinks his formula and follows the diet. Many issues that you might hear of such as headaches, tremors, poor concentration, etc are correlated with poor dietary control. In another words these symptoms are usually only seen when treatment is not maintained. With proper treatment your little guy will grow up to be a happy, healthy, fully functional child. I cannot stress that enough!! He will be able to go to school, join sports, hang out with friends, etc just like any other child, and the only difference will be that he will need to eat a little bit differently. However, as a child my perspective on eating differently has always been 'everyone has something'. There are so many food allergies, diabetics, gluten intolerances, vegetarians, and so on today that eating differently isn't all that uncommon.
As for the cost, the metabolic formula and special medical foods (ie low protein pasta, breads, etc) can be a bit pricey. Some states such as Wisconsin, which is where I am from, provide the formula and staple foods through the state. Other states provide an 'allowance' of some sort to cover a portion of the formula, and some insurance companies will help with this cost if pushed enough. According to this link http://www.pkunews.org/rights/lobby6.htm Washington does assist with this cost. However, you will need to check with your clinic, because I am not sure how accurate this is or if there is a limit on how much help the state will provide.
Like I already said your son is going to be just fine. It sounds like he has wonderful parents who are going to ensure he gets the care that he needs. I was diagnosed with PKU and started treatment when I was nine days old. Regardless of the fact that I have PKU I am a happy, healthy college student studying nursing, dancing on my colleges dance team, working in a child care center and as a dance teacher, and helping to setup a state support group in Wisconsin for PKU. There are so many successful teens and adults out there living with PKU, and I'm sure as time goes on you'll have time to meet some of them either through the internet or local PKU events.