HI, thanks for using healthcare magic
If the damage to the kidneys is due to an acute or short term condition then it is possible that the damage would be reversible and he could regain function.
Acute damage occur in situation such as infections, blood clots
, trauma, dehydration
, inflammation, obstruction.
These are only a few examples.
If the damage is long standing and potentially continuing then it is unlikely that any reversal would occur.
You may want to discuss the prognosis with his doctor.
I hope this helps