How do you control CO2 levels from getting too high?

My husband had COVID pneumonia a year ago which has left him with pulmonary fibrosis in the bottom third of his lungs and inflammation in the rest of his lungs. He does not have COPD; however, his last two hospitalizations have been due to retaining CO2 (first week of November and last week of December). His arterial blood gases in November showed CO2 at 102--very critical level and 98, also critical in December. Used BIPAP in hospital but it didn't seem to help much. Pulmonary inpatient rehab (3 hours of exercise a day) is what really helped to bring the CO2 levels down; however, it is difficult to get my husband to do that much exercise at home. Is there anything else that may help? Both times he came home on 3L of continuous oxygen and not having to increase it with activity. His oxygen needs are slowly creeping up, needing 5-6 L at times with sats at 92-93%.