The answer is "It depends on the type of HDHP coverage you have." I would imagine that since your son is on your plan that you have Family coverage (Family coverage is you and at least one other person). You can confirm that with your HR or benefits person.
In 2018, the annual HSA contribution limit for Familt coverage is $6,900 if the owner of the HSA (you) is under 55. It was $6,850 for a short time early in 2018, but the IRS changed it to $6,900 before tax season started.
