You've been paying a lot more for family coverage when you might be better off with a low-cost plan out of pocket for the kid, and reducing your plan to Individual + Spouse. My wife and I do one individual plan and a Parent+Child plan this year. In 2016 we're going to three individual plans (our daughter turns 18 next year). That lets us pick the coverage that's appropriate for each of us.
When you retire, what do you plan to do for insurance? You could still add him to whatever insurance you get for yourselves next year, although it's probably better to get individual plans for each of you.
Your kid could get his own health insurance through the Health Insurance Exchange in your state (or healthcare dot gov), and you could opt to pay for that out of your retirement funds every year. When does he plan to get a job? Top of the line platinum plans are 500-600 a month here in NY, where everything is pretty expensive. A bronze plan is $200 a month, and at 18 you don't usually need much more than catastrophic coverage.
