Child is High myope.  What is vision of other eye.  Depending on that he should use glasses.... Patching...  And convergence exersices if he develops asthenopic symptoms.

Firstly your child needs refractive correction. Any activity will be helpful only then. You have to patch his left eye (good eye) for at least 2 hours a day to begin with. During this time, the right eye should be used for doing activities like playing. The child will resist in the beginning but you have to be persistent. The results might be slow. The power of the child will not necessarily come down. But his vision with glasses will improve.