I concur with everyone else that it sounds like he's not taking enough digestive enzyme. My boyfriend had Whipple surgery in Sept '07, and the way the dietition explained it to me is that Creon 10 relates to the fact that he should take one Creon 10 tablet for each 10 grams of fat in the meal (there is also a Creon 5 and a Creon 20 tablets, etc). Some of this depends on whether or not he has any pancreas left and thus might be producing some digestive enzyme himself, and that's where the trial and error part comes in. Most people eat pretty much the same stuff 90% of the time and you just learn how much enzyme you need for what your are eating. Most of the time 2 enzymes per meal works great for him, but if he's eating something like fried chicken or fried fish, he'll take 3 or 4. Also things like sausage with eggs fried in butter might require more because of the fat in the meal.
It is also very important that he take the enzyme at the start of the meal, although I think it is also okay to take a second or more enzymes during the meal and if he eats more than he thought he would, I'd take an extra one along the way. The thing is there needs to be enzyme ready and waiting for any fat that is coming through the pipeline.
My boyfriend, age 63, also lost a fit 184 down to about 125 lbs from the time he was diagnosed in April (he had already lost 25lbs by then) while undergoing the chemo, radiation (June & July), and then the surgery on September 7,2007. We were told that he would not start gaining weight back until the wound healed, and they were right. The body prioritizes where it puts the protein that is used to build tissue. Brain, heart, lungs first, then it uses the protein to build tissue to fill in the wound. There were some problems with getting the wound to heal up and I packed an open wound for a month and a half, but once it healed he started putting on weight and has gained 22 pounds since the first of November. He has leveled off a bit now and is gaining much slower, but it has always been hard for him to gain weight.
I would definitely talk to the doctor about his enzymes, or get a second opinion about this because I find it hard to believe that he should be continuing to lose weight like that. I'd say, go with your gut feelings, and I think you feel like there is something amiss with this scenario.
Good Luck,
Donna