Sounds like you putting him to sleep, might be the best option for him, Poor little guy, that is so sad, but you did your best and helped them out, and still have his sister.. after what whispermom2 said, I think that would be your only option,and I'm sure the vet would not steer you in the wrong direction.
I'm sorry you have to make such a hard decision.
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Rhonda, Bailey, Josie and my angel Bella Rue'
"Until one has loved an animal, a part of one's soul remains unawakened."
~Anatole France~