Hello and Thank You for contacting JustAnswer
I am sorry to hear your baby is having trouble.
As you know there can be several causes for the drop in red blood cells and it sounds as if you have done all the testing other then a bone marrow test.Depending on the cause a transfusion may or may not fix the problem. You also noted other problems such as abnormal kidneys(kidney failure is the leading cause of death in cats), low blood pressure, not eating or drinking.
While this really does have to be your decision it sounds to me as if this baby has almost no quality of life at this point and considering all the other problems that the transfusion will not fix it may be best to let him go. I know how hard this is for you I just had to make the decision myself to let my dog who was 18 go.
On top of everything that is already causing problems I am also concerned about his liver at this point. When a cat stops eating the body begins to burn fat the problem is the liver can not process the additional fat and cats develop a problem known as hepatic lipidosis or fatty liver, this can and does lead to liver failure rather quickly if the cat does not start eating.
When are four legged family members reach the point where they are suffering or have no quality of life we have the option unlike with our human family to end the suffering or simply to prevent it. In the end often the last act of kindness we can show them is to allow the vet to help them pass peacefully and painlessly.As much as it hurts it is our ob to be as faithful and loving friend as they have been to us.
Unfortunately I can not tell you what to do but can only offer my opinion based on the information I have been given. It is always best to have a very frank conversation with the vet on what they think the chances are as they know the full medical history.
I hope this has helped in some small way, I will keep you in my thoughts, please reply back if I can be of additional assistance