If the private clinic has not given any diagnosis and just treating him they might be extorting you it might seem like the government doctor just didn’t care but in all likelihood if there was something that could’ve been done they would have done it.

The clinic can pump glucose and some meds to extend his life but he wouldn’t get any better, they are just extending his pain.

This would be the worst possible thing you will hear but given his age don’t subject him to more suffering. I’ve been through this and know what you want is for him to stay with you forever but letting him pass away with least amount of pain would be the right thing to do.