Has anyone here got any experience with secondary cancer?

Could be personal experience or through others?

My dad was diagnosed with stage 3 esophageal cancer nearly 3 years ago and after successfully ridding of it through chemo and radiotherapy, they found one in his stomach that is secondary from the first cancer he had. He’s had SABR treatment (basically a type of radiotherapy that specifically hits one location of where a cancer sits rather than radiotherapy that spreads) they’re worried that the first original cancer will come back as it’s very high risk for that. Although he had treatment on his secondary one, they’re not giving full details on how it’s gone...they’ve said it’s shrunk but not gone and it took a while for them to just be open and tell us it’s still there basically. Precancer my dad was about 21 stone? Now just 2 years later he is about 10 stone. He’s 6’3 and 54. He looks like an emaciated stray dog. I can feel all of his bones when I cuddle him and I’m terrified...he’s losing weight and it won’t stop coming off like melting butter. He’s eating and we’re trying to force in high fats! As well as healthy food. He’s lost 13kilos in 2 weeks and he’s one not far off being hospitalised for being severely underweight. Can anyone please give me any advice on what I can do? If it’s normal after this treatment? Anything?