Hello. Welcome to hcm.
It can be a case of peritonitis
,either subacute or chronic where the fever will be absent. Other reasons might be a localised infection in his abdomen,abdominal tuberculosis
I advice you to take him to hospital where he needs a ultrasound
abdomen first. If it is normal,he needs a endoscopy
or rarely CT scan to rule out pathology and then treated accordingly.
All the best