Over correcting (giving too much) prescription (especially for a child this age) can cause problems with the eyes not working together properly, as can the measurements being wrong. He is still within what is called the ‘plastic period’ where it is REALLY important to have accurate correction for proper visual development.
Which is why, in the UK, any glasses/contact lenses/sports eyewear/safety eyewear being supplied to a child must be sold under the supervision of a registered practitioner.