Hello welcome to healthcare magic, I am sorry that your father is feeling unwell. It is unusual to have a painful throat for 6 weeks. Most cases of sore throat
are due to viral or bacterial infection, in the cases of viral infection ( most common) it tends to resolve on its own, in the case of bacterial infection antibiotics may be necessary. Bacterial infection usually responds quickly to a course of antibiotics
Allergic rhinosinusitis may also cause sore throat if the person has a post nasal drip
( feel mucus dripping at the back of the throat) but this is usually intermittent.
Another cause would be a growth in the throat , smoking
is a risk factor for this.
If he does not have a history of allergies and has been treated adequately for possible infection then he needs to be assessed for more serious causes of throat pain