Based on the symptoms ,it seems your father may have a perianal abscess or a fistula-in-ano, which often needs incision and drainage so consult a surgeon.
If you father is Diabetic it's very important to check FBS. ,PPBS and take medications accordingly and make sure
diabetes is under control.
Next Steps
surgery consultation
Health Tips
presently U can do warm sitz baths ( add salt in warm water in a basin and sit for 15 to 20 mins ) pain relievers like paracetamol, and a high-fiber diet can help provide temporary relief. Delaying treatment may worsen the condition, so I recommend seein a surgeon immediately