I think it depends on how unwell he will remain into his adult years. I have a sick kid for the last 6 years, not cancer but being treated with cancer like drugs and not going to be cured, but hopefully not fatal. The kid got a medical card a few months after diagnosis, and has had one since. But we have kept on the health insurance because life is very different in adult services. Getting a scan, a test, to meet another consultant could take forever in the public system and I have no issue going private when the child needs the next step done. We were treated as private patients until she got the medical card and are now treated as public, but one time she needed to move hospitals while an inpatient and we were asked by the doctor if they could use a private ambulance for the transfer, so she was changed to private on the hospital system. In the end she did not need it because the consultant said we could drive her ourselves. But next time we were back to the local hospital she was still down as private and there was a bit of a palaver to change her back to public.
So if you can afford it I would continue to pay because I doubt if the kid would be covered if you wanted to restart cover at a later time due to "pre existing conditions". In reality you are paying for future cover not current need.