I have to agree with Niuniu. Phe tolerance changes a little bit throughout life, because so many factors can affect phe tolerance (ie growth spurts, activity level, etc). Your son is still very little, so he will be going through a lot of growth spurts in the near future which means his phe tolerance will change somewhat here and there.
When I was younger my phe tolerance was 300 mg of phe, however at one point due to a growth spurt my tolerance was increased to nearly 420 mg of phe for a few months. When I was done growing my tolerance was brought back down to 300 mg of phe. I haven't heard of a case where someone was suddenly not able to tolerate any phe at all.