Replace regular milk with Fairlife super-filtered milk (or other similar milks) which has 2x the protein per serving as regular milk. At least he'll get more protein that way. Does he like bacon or sausages? My wife wouldn't touch those fatty breakfast meats but in order to get more protein into her breakfast she'd get a piece of turkey bacon or chicken sausage. That's 7 more gms of protein without eating a huge volume of food. Vegetarians supplement protein in various ways, one of them is Bragg's Nutritional yeast. It's 5gms protein per tablespoon. I hide a tablespoon or two in my wife's mashed potatoes or cereal, food that normally don't offer much protein. Your oncologist should be able to refer you to an Oncology dietician or nutritionist. Don't delay. Excess weight can disappear fast when you're battling pancan.

Good luck.