Asophocus cancer

Hi my son who was 44 when he got diagnosed  November 2022 with asophocus cancer unfortunately  a cat scan showed it has spread to his liver. He had a tube fitted for his feeding as the tumour was blocking his asophocus. So he couldn't eat or drink . The tube was OK on n off kept blocking resulted in my son having no food only a drip on n off for 10 days .then he got fitted with a stent  he started to put on weight .as he had lost a lot of weight by not being able to eat or drink.we were all pleased he was now putting weight on . My son has been eating and drinking really well and build up  shakes. He had put nearly a stone back on in a month .now to my dismay I weighed my son today .he has lost weight .which I can believe as has been eating so well. He is starting  his chemotherapy  treatment  next Monday 13 February. I am increasingly  worried that it's the cancer  that is causing the weight loss. My son needs to build himself up for the chemotherapy  treatment. all wad going OK now this step back again. He was 12 stone before the illness struck but went down to 9st .because being unable to eat . Any insight  would be appreciated  thanks