I'm sorry to hear that Marley has been diagnosed with a mass on radiographs.
Is the mass in his abdomen but so large that it is pressing on his diaphragm and thus compressing his lung(s)?
I understand that he was originally seen because of a cough.
Dogs that cannot breathe and exchange oxygen normally do not eat well. The body prioritizes oxygen exchange over eating and suppresses their appetite.
But of course with a mass he needs the calories.
It may be easier for him to swallow liquids or soft foods. I would recommend a soft prescription food called Hills a/d. It is very palatable, easy to swallow, and is extremely nutrient concentrated. We often feed it after surgery, in dogs with cancer or serious illness that need the nutrients and calories.
Anther option is a liquid formulated for very sick dogs and cats called Clinicare.
Ensure is formulated for people, so it isn't the best option, but you can try it to get some calories into him while you are trying to obtain the other things I recommended. Just make sure the formula does NOT contain xylitol as a sweetener, as that is highly toxic for dogs.
Make sure he gets plenty of rest too, as the more he moves the harder he works to exchange oxygen and the more calories he burns un-necessarily.
Please let me know if you have any further questions.