I had a tough time swallowing vitamin supplements in pill form, so didn't really use anything like that prior to, following surgery, or during the time period that I was getting radiation. What I did do, was use more whey protein (either just mixed with water, or added to a milkshake), and use my juicer more often. It's very difficult to get your vegetables when your mouth is in this state, so I supplemented my diet with veggie/fruit juice. I scoff at the Fat, Sick and Nearly Dead guy and his reliance on juicing ONLY for his daily caloric requirements, but I do believe that adding juice to your diet (in conjunction with as close to a normal, balanced diet as possible) is quite helpful. Now that I can eat mostly everything again, I don't use my juicer as often, but still like to crush up a nice green juice whenever I realize I haven't been getting enough veggies lately.

Eating enough calories and the correct balance of nutrients will go a long way to maintaining a decent energy level. If he doesn't already know, calculating his "maintenance calories" will give you an idea of how much he should be eating. For a moment, I'm going to ignore the effects of the cancer and assume he's healthy, and NOT needing to put on weight. The numbers change based on activity level, but I'll assume for a moment he's not all that physically active at this point. A 37 year old man at 6' & about 170 lbs needs approximately 2100 calories. If he exercises very much at all, that number goes up. When I was starting to look at cleaning up my diet, cut some fat and add muscle a few years ago, I found this calculator to be helpful. Along with the knowledge that your body needs an approximate breakdown of 20-30% fat, 20-30% protein and 40-60% carbohydrates, I was able to understand what my body needed for fuel for me to reach my fitness goals. For tracking calories, I didn't really go overboard, but found the "myfitnesspal" app to be quite useful as a learning tool. After tracking for a few months, I was all set and knew how much I needed to eat (or sometimes not eat) based on how many times I was on the water that week, and how intense my practices were. Since I was trying to add some muscle, I tried to add maybe a half a pound to one pound per week. One pound is 3500 calories (well, that's disputed but the number is close enough), so I aimed for an extra 500 calories per day above maintenance. I believe that my fitness level helped make my recovery go a whole lot quicker than what it could have been like.

Now earlier I said I was going to ignore the effects of the cancer. This process of the surgery and subsequent treatments is going to be very physically taxing, and he'll have to consume even more calories in order to maintain weight and feel reasonably energetic. I didn't find any concrete numbers I could plug into my formulas that suggested how much extra energy is expended/required, so all I did was estimate what I was eating, and compare to what I saw on the scale and in the mirror. I'd guess I added maybe 1000-1500 calories per day on top of my regular maintenance calories, while I was undergoing radiation treatments, and managed to keep my weight fairly consistent and not feel too depleted to get outside for a 20 minute walk, or something like that.

Edit: Oh, should also mention that the old idea of sugar feeding cancer is somewhat of a myth. Here are a few sites that suggest otherwise: Canadian Cancer Society and MD Anderson