I would think to prescribe an antibiotic, the vet must suspect an infectious cause of your dog's spleen enlargement.
Ask the vet if he/she has dicovered an infection, or why they suspect an infection. The clue should be in the bloodwork, but they may have run other tests.
They should not just be prescribing from guesswork, to see if antibiotics work first, before investigating further.
Splenomegaly can often be caused by infection, but of course, there are other causes.
It may be that an abdominal ultrasound will be needed.
Anyway, I hope the amoxycillin does the job, and that your dog will recover well and feel better soon.
If you are trying to encourage appetite, I would suggest not feeding kibble, but something like a plain chicken broth (no salt) with a few vegetables. That is much more enticing, and will also provide hydration.
I am so sorry to hear this. Eating is something a dog can do for a bit but not drinking is problematic. Can you do a dropper of water into the mouth? What did the vet say? A lot of things can cause fluid in lungs. They are treating with antibiotics and bacterial infection is one of the things that can cause fluid in lungs. Infection can also cause an enlarged spleen in dogs. It sounds like your vet thinks your dog has an infection. Cerenia is for nausea in your dog. Keeping your dog hydrated is your most important task right now. You could try flavorless pedialyte that has electrolytes in it to help with hydration. Leave water bowls around. I've offered water from my hand before and my dog will take it even if not feeling good. How is your dog doing today after you posted this?