He will need a complete examination as there can be a number if causes for sudden decrease in vision. However in the absence of any acute symptoms like pain or redness the cause is more likely to be chronic or slowly evolving and less likely to worsen in the next few days to weeks.
Given the present unfortunate circumstances these days and given his age, it is best to avoid getting him into any public space in the near future unless there is an emergency.
Next Steps
Keep his symptoms under close observation and schedule a clinic appointment for a complete examination once the present global healthcare situation is under control.
Please continue all existing medications if any and get him to take adequate bed rest and hydrate well for now.
Please feel free to get in touch in case you need any further assistance.
Hope this helps.