No bladder is going to permanently hypotonic in 12 hours, even in few days as well, it will recover in couple of weeks. If he is not able to void then he will required CISC- clean intermittent self catheterisation, that is standard treatment of this condition. For how long he will need to do CISC, no one can predict. But many patients recover in 1-3 months.
Next Steps
Need Urodynamic study if not done to check bladder pressures