Possibility of covid cant be ruled out on only RTPCR negative.. Do he have any respiratory symptoms along with fever or any other symptom in body.
If any respiratory symptoms are there than go for HRCT Chest which will be more diagnostic.
Next Steps
share all symptoms via online video consultation or via chat
Health Tips
Isolate ur father in a room and all people having no symptom should stay away.
wear mask everytime when going near ur father