I had lost 30 pounds (diet + exercise + cancer) prior to my Stage-2 PDAC diagnosis, and managed to regain it all while on Folfirinox for 6 months. Lots of meat/protein/fat in my diet. Heavy whipping cream in my coffee accounted for about 800 calories per day.

After 6 months of chemo, I had the Whipple, and lost about 15 pounds in the following 4-6 weeks, but have been stable ever since. I had to start Creon and cut back on the fats, but even after 4 months on a new chemo since my recurrence, my weight is still stable. Still eating lots of protein.

If your husband is not eating enough, then he needs the most calorie-dense nutrients he can get. Smoothies and shakes might fit the bill. Patients also absorb less nutrients after a Whipple, so there might be some other vitamin/mineral supplementation required beyond just enzymes. I was getting vitamin B12 injections for a while to fight fatigue (and low B12), which gave me enough energy to want to eat.

For appetite, my dad has been taking megestrol during his treatment, and eating like a horse. You might be able to get a prescription for medical marijuana to stimulate appetite, but your access and mileage may vary. I've found the non-psychoactive CBD gummies and oils helpful in calming my digestive tract. I can't say they stimulate appetite, but are a little bit calming and relaxation-inducing and nausea-reducing, which makes eating just a little more pleasant.

The psychological aspects of past nausea can also induce fear of eating in the future. Reflux was an issue for me while on Folfirinox, so I had to make sure I didn't lie down in bed too soon after a meal. I would often nap sitting up in a recliner after meals, and avoid drinking too much fluid with meals to prevent food from sloshing around, and focus on thicker foods that "stayed down" easily.

Wishing you both the best!