Hello i have noted your concern,
Any spo2 below 95% is not normal, even during sleep spo2 will not drop to 90% in healthy individuals so my opinion is,
Your father may be suffering witH, consult a pulmonologist physically and get one sleep study done, if he have osa, then he may require to use C-PAP machine at bedtime regularly.
Next Steps
Kindly consult if you have any further queries