I'm completing my son's taxes. I'm at the part where their asking about the 1095-A. my son is 18 currently on my health insurance do i select yes or no for healthcare.gov

It depends.

If you are claiming him as a dependent then answer no.  

if you are not claiming him as a dependent then answer yes.  Then you will need to choose that you shared the policy with another taxpayer and allocate the percentages.  You can choose any percentage between 0-100 as long as it adds up to 100%.