You ask what you are doing wrong. You are not doing anything wrong, this is a nasty disease which is very individual and doctors often don't even treat it that well so you should not think that you can cure him.
I would ask about
biologics as others have said.
What sort of iron is he on 3x a day? Iron can be quite toxic for UC, which does make treating anemia complicated. Iron infusions are more recommended than pills for severe anemia.
The immuran may be adding to his exhaustion, it unfortunately causes tiredness for many above the awful fatigue that UC causes. UC does come with awful fatigue, he will need to rest and accept this
www.fatigueinibd.co.uk/He can have a stool test called fecal calprotectin to try to judge current levels of inflammation without putting himself through a scope.
The reason you go to the bathroom so quick with eating is not down to the type of food, it is that your colon is inflammed so any chewing wakes up the digestive system and then the inflammed parts try to move on anything touching it asap, they cannot do the normal work of the colon which is to absorb excess water, hence stools are loose, although some parts of the colon are sluggish too, which is why you can get mixed constipation and diarrhea.
Tramadol is a strong, addictive medication and can upset the bowel in itself too. Does he really need to be on it daily? He may well be addicted now. I know that some with IBD may have terrible pain if they have strictures or fistulas but with "normal" UC the pain can usually be managed without daily opioids. Could he try learning the practice of mindfulness to deal with his pain?
He came off Lialda you said but could he tolerate rectal mesalazine?