My cat won’t eat

For background, I have 2 cats that I picked up from the side of the road like 2.5 years ago. Both of them are male and approximately 2.5 years old, one is named Nemo and the other is Loki, both were neutered in early 2022.

I went on a 10 day trip (left a cat sitter at home) and came back last week to 2 cats who had lost some weight. I changed their food and while Loki ate and gained the weight back, Nemo didn’t. He seems less active than before as well. I tried giving them tuna as per vet’s advice, but Nemo just smelled and licked the food a bit then left. I’m getting really worried bc I’m not sure if this is just because he doesn’t like the food or if there’s something wrong with him that’s causing him to not eat. I’m thinking of taking him to the vet tomorrow but any advice on here will be helpful.