In 2019, the tax penalty will be gone.
I am "guessing" not claiming step son as a dependent, thus not obligated. The (prior) law (allowed) you to cover your children, step children till age 26.
But, one thing you should consider (before) you drop him, will your insurance premium change? I "think" with employer insurance, the cost is the same for 1 child or 10 children. So, if no drop in costs keep him on the policy "just in case".
And lastly, what if something in fact happens, will your husband, his father help him out or not?