Thank you for your reply.
Nearly every California child support order (like other states) has a provision for health insurance, but health insurance coverage is separate from child support. There is no law that says because you pay child support that you must pay for health insurance. In fact, the parent responsible for providing insurance may not be the parent paying child support. It depends on the circumstances.
If either parent has health insurance coverage through an employer, and health insurance coverage for the child is available at little or no cost, typically that parent will be ordered to maintain health coverage for the supported child. The parent may be held in contempt of court if he or she cancels the child’s health insurance. So for example, if you were providing it and then took it away for no reason, you could be held in contempt (the same for your ex, if they had been the one providing support). I understand what you are saying about not having been able to see your son - and that wasn't fair to you - but from a court's perspective, health insurance, like support, has nothing to do with visitation, it's for the benefit of the child, so taking it away only punishes them. However, if the reason you took health insurance coverage away was because you switched jobs and coverage for your son wasn't available or was too expensive, then that's different, and you cannot be forced to continue to carry it. In that case, typically, again, what a court order would say is that should circumstances change for either parent and it becomes available to a parent, then that parent must carry the insurance. The court may go so far as to order that in the interim, where neither parent has coverage, the child's medical costs are split evenly.
All that said, you don't say in your initial question whether the coverage simply became too expensive for you to carry or was unavailable to you during those years in question. Arguably, even though there may have been nothing in your court orders regarding insurance, and it's not a requirement of a parent who pays support, because you were covering your son, if you were still able to do so (and you say July 2015 to September 2018 so I assume you wrote the wrong dates) and chose not to continue the coverage, a judge may take issue with that and hold you responsible for the medical expenses incurred.