Oh im sorry to hear this. {{{{{Midnite}}}}}
Looking at an older thread you made, I see you mentioned the biting and sexual behaviours before. I does make me wonder if he has something affecting his adrenal or pituitary glands. I suppose the biting could be a bit of a coping mechanism if he does have some discomfort.
Does your vet think he would need any medication to deal with any possible pain due to the tumour? Or to suppress the growth?
How has the vet attempted to draw blood so far? Getting it from the back leg vein can work well. I saw a vet do this for my older rabbit and it was quite effective. She just shaved a bit of fur off the outside of his leg, then popped his leg just off the table while I held him still (other 3 legs on table). The gravity helped get the sample.
At 12, I would focus on just making sure he remains comfortable, if it were my rabbit. I wouldn’t want to put him through biopsy either.