Many owners will not realise their dogs are going blind, as the gradual process means they get used to their condition and can find their way around using their other senses. Its only if the blindness is a sudden onset that it can be very traumatic. I would take a 'wait and see' approach, see how the insulin helps him, and if he is well in himself. Only then can you make an informed decision, and if his quality of life is not as good as you would want for him, you will know what you have to do. Its a difficult time, but you will know when the time has come, and as I always say, better a day too soon than a moment too late.