Has he had an ultrasound scan of his entire abdomen? If not, that is what I would push for. I have had it done when I had gallbladder problems. You never know, it could be a tumor or an obstruction.
Thanks for writing to the forum!
Lower right abdomen pain is commonly due to appendicitis. The pain, can at times come and go and not give the classic picture. Crohn’s disease, diverticulitis, infection in kidney, kidney stones, constipation, and an abscess in rectus abdominis muscle or psoas muscle can all cause lower right abdomen pain. An ultrasound or CT scan of abdomen is definitely required.
I would strongly recommend you to see a gastroenterologist (a specialist who looks after the diseases of our digestive system). It is difficult to comment beyond this at this stage. Please let me know if there is any thing else and do keep me posted. Take care!