As he has recovered from covid, our body takes time to restore back to normal state. Just give it some time along with rest, nutritious food small in quantity at frequent intervals which may include light food items like fruits, sprouts, juices, salads. Drink plenty of fluids. Have dry fruits in small portion daily. Try eating more of citric and fruits containing fibres.
Next Steps
In case the conditions persists for a longer time, get consulted by a doctor.