Sick bunny....

Yes.. I have a bunny. I got him on my 21st birthday. he's an OLD bunny.. saw him today breathing hard, and noticed he hadn't had much to drink or eat.. took him into the vet, and he's got a hairball, and pneumonia.. it doesn't look good. they want me to force feed him, and I do, but most of it just falls out the other side of his mouth. He's got so little fight left in him.. I just put him back in his pen, and he fell on his side, and while he struggled, he couldn't upright himself.

as always.. the question is, when does it become better for him to put him down?