If he can eat and digest fats well, you might try adding some food high in healthy fat to his diet, like peanut butter or almond butter, avocado, olive oil. I don't know what he's able to eat because of his situation. Fats have over twice the calories per gram of that of protein or carbohydrates. It could be a good way to add quality calories without adding much quantity.
It's impossible for someone out here to say why he's losing weight. For an guy with a body that's battling illness, 2,000 calories is not really that much. He can follow his doctor's advice as far as physical activity. He could be losing lean muscle mass. You could also consult with a dietitian. I have found on the internet that the American Cancer Society may have a dietitian available:
"Cancer patients often face challenges with eating due to treatment side effects and may not get the proper nutrition in order to maintain their health. As a result, the American Cancer Society offers free nutrition counseling by registered dietitians specializing in oncology. Our dieticians provide patients and caregivers with personalized nutritional counseling by telephone. Dietitians also send written materials and provide follow-up to continue the support when needed. After treatment is finished, dietitians provide assistance with strategies for a healthy lifestyle. Contact us at (1-800-227-2345) for more information."
They may be able to advise you on the special dietary needs of someone with cancer, or recovering from cancer, especially since your husband is taking part of his nutrition by feeding tube.
You really need to talk to someone who is familiar with your husband's overall condition-- his doctor. They will know about any problems related to your husband's cancer, and also his circumstances with the feeding tube. I have no experience with feeding tubes.
In the case of my father, who has an advanced cancer, I am watching for a condition called cachexia, in which he would still lose weight no matter how much he eats. His weight loss has stabilized in the last three weeks, without forcing him to eat. He lost almost 19% of his body weight within just a few months.