I just posted about my Freddie.
His started the same way, lack of appetite and weight loss.
X-rays showed a mass so we were referred to a surgeon. The ultrasound this morning shows his spleen is completely gone and most of his liver is as well.
Yesterday the vet said if the liver wasn't too bad he could remove part of it and then consider radiology or chemo. Freddie's lungs were clear which was a good sign. Unfortunately, the other stuff wasn't good.
I would suggest a second opinion. Freddie is nearly 15. Your baby is 6. I'm not saying your vet is not right, I'm just saying it sounds too gloom and doomy to me without at least an x-ray/ultrasound.