My child (15) blood sugar level is 21.7 at the moment. no doctors available. no insulin available. what to do?

This is an emergency: This is a very high blood sugar level. If it has been building for a while, you will need to have his insulin adjusted. I am assuming your child has a diagnosis fo diabetes. Until you can access care, you should have him take fluids (non sugar containing electrolyte fluids at first ) as much as possible. I would estimate he would need at least 2-3 liters to help bring down his sugar. Try to get him to care as soon as possible. As you are providing hydration, you will see the blood sugar come down and then you should replace the sugar free fluids with those containing glucose and electrolytes as oral hydration.