My son has native accent in Spanish since he learned so young, why? Does this also come later in High School?

I really wanted my son to learn a second language in preschool.  I decide on Spanish since my husband and I both took classes in school and have some knowledge of words.  We decided on an immersion preschool for our son, and he went two or three days a week.  At first it was hard for him as his teachers spoke only in Spanish, only using English rarely.  Though it was difficult to watch, I knew from my own experience as a teacher, that he would learn quickest and sound more like a native, from an immersion model.  My son now has a natural accent when speaking Spanish, though he still isn't truly fluent.  It takes people by surprise!   I looked up accents on wikipedia and this is what was written:

"The most important factor in predicting the degree to which the accent will be noticeable (or strong) is the age at which the non-native language was learned.  The critical period theory states that if learning takes place after the critical period (usually considered around puberty) for acquiring native-like pronunciation, and individual is unlikely to acquire a native like accent.  This theory, however, is quite controversial among researchers."