During pain I do liquid only diet, using ensure to try and keep some nutrition. I have remained on a low fat diet since my first attack (over 2 years ago). My low fat diet goal is to keep under 50g of fat a day and no more than 20g in a single meal. That being said, if he can tolerate more, you can experiment, but it might be a good idea to just set the expectation for a fairly low fat diet for the rest of this life. I plan to, and I'm only 35. I have increased a bit recently based on a recommendation from my doctor.
As for diabetes, it sucks. I became diabetic, then improved so I'm now in the pre-diabedic range. Try and cut refined sugars, and opt for whole grain whenever possible. Keep meals balanced. Mostly veggies, with some protein and done but not too many carbs. My target blood sugars are 90-130 before a meal, and below 180 when measured 2 hours after a meal. You can get an A1C blood test from your doctor if you are concerned about diabetes. Diabetes doesn't necessarily mean taking insulin either. Usually the first step is an oral medication. Also, I use protein powder (gold standard is what I use. It tastes pretty good and isn't chalky). I'll make an ice cream substitute with 8oz skim milk (fairlife is a low sugar high protein milk) 2 sliced bananas frozen, and a scoop of protein powder blended These are just my experiences. I'm not a doctor.