My thoughts were, like foufouka said, it will tell us how much phenylalanine he can have. Taking the guess work out of the diet. I hadn't given any thought to if it meant which source of protein.
They originally thought he was HyperPHE, however, now say he is Mild PKU, which again, from my understanding, are slightly different. This test will give the us the answer as to what exact form he has, so we aren't worrying that it changed once, will it change again.
From my understanding it can also let us know if medication (like Kuvan) would be an option available to him. Would his levels be reduced by it, and cause less of a restriction on food. (This can also be done by another test which checks Phe levels before and after administration of BH4. He had this test early on, although the drug is not being approved for him at this time, we are aware this will work for him in the future should we decide its needed.)
We will likely know and understand more once we get the results. It was explained using broad terms of will help with what diet restrictions are necessary, it will help us in knowing how to best treat Jacob. I am almost embarrassed to admit, that because to us, it was little more than a draw of blood for Jacob, we agreed almost blindly. I can see how you would want/require more questions to your answers when you have to factor in dollars, expecially when the costs of feeding PKU children can already be a struggle.