Hi, yes there are many other potential reasons for gastrointestinal bleeding
. If he had an ulcer seen by endoscopy, he is at risk of having ulcers in his "gastric remnant", which is usually not seen by endoscopy. The gastric remnant is the piece of stomach that is cut and disconnected, but is left inside the body, and still produces acid, and ulcers can happen in that part.
The only way to diagnose it would be with an "enteroscopy", which should be done preferrably with a "spiral enteroscopy" or "balloon enteroscopy".
There are also many other causes of bleeding, which are usually seen by endoscopy, or by a capsule endoscopy
Hope that helps, let me know if you have any further questions.