Hello!
This is a very rare combination of diverticulitis of the sigmoid colon and arthritis. There is no name of this disease or reason for why it occurs. Very few cases have been reported and documented in medical history. As he has already been cured of diverticulitis by surgery and all his parameters tested near a Rheumatologist are negative clearly suggest he does not have any auto immune disease.
You should see an Orthopedician or Rheumatologist and continue his symptomatic arthritis treatment and follow up regularly so that it decreases completely.
He should be careful while using methotrexate, as the reason for arthritis was not known and should take folic acid along with it and with any adverse effects should stop it. The response to the prescribed drugs should be monitored closely. Try using ice packs regularly for the inflammation of the ankle joint.
I would like to quote: abstract the copyright 2001 of S.Karger AG, Basel :
BACKGROUND: A causal association between acute diverticulitis of the sigmoid colon and arthritis has rarely been reported. CASE REPORT: We report the case of a 60-year-old patient who developed migrating arthritis of the knee and ankle during the recurring episode of acute diverticulitis of the sigmoid colon. Treatment with NSAIDs and antibiotics had little effect on joint disease, but medical treatment was successful in reducing the diverticulitis-related symptoms. Arthritis promptly improved after surgical resection of the sigmoid colon, and 30 months later the patient is free of symptoms in the previously affected joints. CONCLUSIONS: Five cases of diverticulitis-associated arthritis have been reported. The similar case reported here reconfirms that joint disease has a limited response to medical approaches. Colon resection is recommended for patients with diverticulitis-associated arthritis which does not respond promptly to antibiotic therapy. Copyright 2001 S. Karger AG, Basel
Take care!