Can you EDIT him in the Personal Information section and make sure he is a dependent and the date of birth is correct. If the one child qualifies, then the son should also.
Yes, you can amend 2015, but get 2016 straightened out first.
In the year of birth (2016) you have to say the child lived with you all year - as it is referring to since born.
To Amend the 2015 return see below:
https://ttlc.intuit.com/replies/3288565
