It's diet. If you still eat fatty things your liver will still dump bile. This time though, it has nowhere to go but directly into his intestines. After a while he could develop stones in his liver or the ducts leading from the liver which will cause much pain. Also if he has to run to the potty after eating something really rich and fatty that will also go away if he changes his diet. Change is hard but it's better than living in pain. Someone asked a similar question just a few days ago. I'll copy and paste my response:
"Even if you had a gallbladder out you can still get stones that form from the bile the liver produces. The gallbladder was just the storage sac that the liver dumped the bile into so it was the usual culprit for stones and pain. It regulated how much bile to release depending on what you've eaten. Without the gallbladder there's no storage or regulation, the bile just gets dumped. I've also had my gallbladder out and I try and eat a diet low in fats so I can prevent bile dumping. It's really terrible. To quote a website about gallbladder surgery:
"One of the side effects of gallbladder removal can be the dumping of bile which is now not as easily regulated and can send someone running to the bathroom immediately after eating. A more common side effect is a decrease in the secretion of bile. If the bile produced by the liver becomes thick and sluggish, painful symptoms and bile stones can occur. Bile stones can form in the liver as well as the gallbladder. One woman had her gallbladder removed only to end up back in surgery again two or three days later where they found stones in the bile ducts of the liver causing her a lot of pain."
So if you're still having pains in generally the same area. Get an ultrasound to check for stones in your liver or ducts. You also may need to drastically alter your diet to prevent bile sludge in the future."