COPD Dropping Oxygen levels

My dad's a COPD patient since past 8 years.
A year earlier, he could slowly walk for around 2 km in the park. His Oxygen levels (measured using Oxymeter) were around 95-96%
Recently (past 4 months), he's not able to take even 2 steps. His Oxygen levels normally are at 91-92% and drop below 90 whenever he walks even a few steps.
He's had minor infection (cough and congestion) which refuses to go away even though he has taken some very heavy antibiotic injections. The color of his cough is mild yellow.
Please advice what can be done here. Can it be controlled at this and can his oxygen levels be restored to his normal 96-97%?
Presently he's using the oxygen concentrator for close to 5-6 hours a day and is on inhalers.
Doctors mostly prescribe some steroids for him whenever he goes for a check up but nothing seems to be helping.
Thanks in advance.