If you think it's mild pancratitis, he should follow the same diet he did when he had pancreatitus in the past, and then slowly add back also using digestive enzymes
to give his system some help.
In autoimmune pancreatitis
, once diagnosed the treatment is basically steroids/corticosteroids which can help.
And since he's just had his GB out, he shoudl be following a low fat diet. In many individuals, they can have problems digesting fats due to a lack of bile release 'regulation.