Moaning sounds like pain. Staggering is certainly not a good sign. A "funny bark" (do you mean hoarse-voiced?) could mean he i finding it hard to breathe properly,or the act of barking is actually painful either for his throat or for his abdomen or lungs.
Don't guess any more Please do take him to the vet as quickly as you can.
OK where is the tumor?
What sort of tumor does the vet think this is?
False hope is not where you need to be, but you need a lot of info. about this tumor, as so many of them are either benign, or even if not, can be removable.
Of course it all depends on the position of the growth, and if it IS malignant, how far has it spread? Also what options are there after surgery, for any treatments?
Don't be afraid of putting him through surgery if there is any chance of it helping him, or if your vet thinks surgery could be an option. Even if he isn't a young dog.
My dog was given a 50/50 chance of survival last year for major surgery, and she made it through just fine.
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