http://www.ag.ch/bildungswege/de/pub/kindergarten.php
On this page they discuss the cut off date (30 April) for entrance into (primary) school. Putting this together with the statement higher up the page that children can visit kindergarten for at least one, also 2 years, prior to school, makes it appear that your son can attend kindergarten in the year he is 4 going on 5 as long as he makes 5 before the cut off date of 30 April during that first kindergarten year (which it sounds like he does).
I am surprised to read that kindergarten in Aargau is voluntary, so you do need to check with the Gemeinde that 2 years of kindergarten are permitted in your new village, as presumeably as it is vôluntary, they are not obliged to provide 2 years.