Then your child is already British.
The reason is that for the first 3 months, there are no restrictions on what you do in the UK. After 3 months you are required to exercise treaty rights in order for your residence to considered so assuming you worked continuously for (almost) 5 years, you would have obtained PR in Dec 2012 (5 years after moving to the UK). PR status is obtained automatically so as your son was born in the UK after you have obtained PR status, he is British from birth. You can apply directly for a passport providing proof of your residence for 5 years before his birth (5 P60's). It would be advisable attaching a cover letter explaining your situation to the examiner.
If you wish to apply for BC for yourself, you can do so from December 2013 assuming you meet the
standard requirements.