Health Insurance for kids over 18

We currently provide insurance for my 4 stepchildren. One is currently over the age of 18 and in college, so we are not legally required to provide his insurance. We do so because we can cover him at no additional cost.

What would our responsibility be financially if he had a medical treatment under the insurance and did not pay the bill? Are we responsible since we provide his insurance?

No, he is legally responsible for himself.