First off, this bill was introduced in the last legislative session. Since it was not voted on, it died on 12-31-10. However there is nothing to prevent a new bill from being introduced. In the case of your son, I would have to believe that a grandfather clause would be put in place for kids already enrolled in school. 60 minutes recently aired a story about parents purposely red-shirting kids who were born in Sep, Oct and Nov. Children born in those months will start kindergarten at 4 and high school at 13. The book "Outliers" suggests that kids born in the first quarter (Jan, Feb, Mar) are far more successful in academics, sports and social skills than those born late in the year. The kid born in Nov, who starts kindergarten at age 4, will be in the same class with kids who will be turning 6 soon after their 5th birthday. In those early years this is crutial. This also carries over in the later years. Some kids turn 16 in the 2nd half of their sophomore year while others not until the 1st half of their junior year. At 16 you get your drivers license. This a huge advantage to the older kid who wants a summer job, not to mention dating. This is why many parents in the 60 minutes piece red-shirted, or purposely held back, their kids if born late in the year.
Illinois' cut-off date is September 1st. This prevents anyone from starting kindergarten before age 5. Kids most likely to be held back are those born late. If you move our date back to Sep 1st, that will be avoided. Better to move an older kid ahead a grade and BOOST their self esteem than hold back a younger kid and lower it.