I am sorry to hear about your situation.
You have my deepest sympathy and support.
First, we need a basic blood and urine test for an overall health assessment.
are not that common, and just about an systemic (internal) illness or even a urinary tract infection REALLY could manifest very similarly.
I DEFINITELY would not assume a brain tumor. There very well could be medication to treat an underlying condition, to return him to normal health.
Your vet, on examination, will likely be able to judge whether they feel your pet is suffering and has a poor quality of life or not.
Nausea and ill health can mask true pain, so its difficult to comment on without having the results of some basic tests.
When to put him down?
It's all about medical management. Once the basic tests are done, the vet will recommend some drugs. They could be anti-inflammtories, or pain medications, or antibiotics if any infection is found. If things improve. GREAT!
If things do not improve, then the prognosis is not good and euthanasia would be considered.
But, not until a vet examination and at least a basic test or two, would I make that decision.
I wish the best for both you and your pet. Please let me know if I can be of any further assistance.
I truly appreciate positive feedback if my advice has been helpful or informative.