Your sons hearing test shows 70dB to 92dB how much does he have?

Your question contains a misunderstanding. Hearing test results are typically reported in decibels (dB) of hearing loss, not absolute sound levels. A hearing test showing 70dB to 92dB indicates the degree of hearing loss your son is experiencing at various frequencies, not the amount of hearing he has.

When someone says 70 dB to 92 dB the meaning should be:

A loss of at least 70 dB at the worse threshold measured 4 frequencies tested - but no greater than 92 dB.