This is so personal and so difficult but I don't think I'd let him go at this point...because the cat is healthy. Yes 9 to 8 pounds is a loss of weight, but it's not deadly at this point.
I've always liked and shared this article when trying to think about such decisions.
http://www.pets.ca/dogs/tips/euthana...e-pet-tip-228/
I wish you much strength