If there is blood emminating from your cat's mouth with out a specific source of bleeding, then he may have a coagulation disorder, where the clotting mechanism of the circulatory system does not work properly. This can be the result of autoimmune disease
(disease where the immune system attakcs the body itself), certain types of infection, and cancer.
If a CT scan or MRI is not fincancially pheasible for you, I would attempt empirical treatent at this point, focusing on stabilizing the patient, quelling possible autimmune disease, and focusing on infections that may be implicated in this case.
First, I would get a whole blood transfusion ASAP. I start transfusions once they reach 12% on the packed cell volume, so by my criteria, the time for trafusion has already come. A whole blood transfusion would also provide clotting factors which may help toward stabilizing the patient.
Next, I would cover with either tetracycline or doxycycline, good broad spectrum antibiotics that are effective in managing many infections implicated in causing clotting issues. At the same time, I would start the patient on maintenance immune suppressive therapy with steroids in order to keep the immue system in check in the event that this it is the result of autoimmune disease. If there is underlying cancer, although this would make the prognosis poor, steroids also have strong anticancer properties, sometimes offering shortlived, but effective remission.
Please feel free to print this out and discuss my recommendations with your vet.