As per the latest guidelines from Education Ministry, they have changed the age criteria to 1st October to 30th September Every Year.
For Example: For Academic Year 2019-20
1st October 2015 to 30th September 2016 For Nursery or Montessori 1
1st October 2014 to 30th September 2015 For Jr Kg or Montessori 2
1st October 2013 to 30th September 2014 For Sr Kg or Montessori 3
1st October 2012 to 30th September 2013 For Grade 1
And so on for higher grades....
In your case, your child is born between 1st October 2014 to 30th September 2015. i.e. Jr Kg. is suitable for son as per the guidelines.
But you can enroll your ward for Sr Kg showing previous year academic records. Most private schools doesn't bother about this issue. But rule is rule. You can try to convince the management.
I hope this helps.