i dont' know much about that chemo regimen, but mine totally caused me to lose weight, and i lost quite a bit but never crossed over to a scary amount of weight loss. i would recommend asking the oncos about this at the next appointment. pretty much, i was told to do what it took to eat more than i was eating. anyway, people on his medical team might have some good ideas... good luck - i hope he is able to eat a bit more soon!
ps - consider adding a bit more plant-based fats, like avocado and olive oils etc... also, i hear fish oil can help keep muscle mass.... there's also a vegetarian version of fish oil, which i started taking when my weight loss got to be worrisome for me. i feel like that helped.