I would consult a behaviorist and some animal sanctuaries like Best Friends before you make any harsh decisions. It could be that someone out there will be willing to take on this difficult case. I would use putting him down as a last resort. The way I see it, you might as well make an effort contacting people and trying to save him before putting him down. It could be that some trainer wants a dog to rehabilitate, or something similar, you never know. Chances aren't great, but it's worth a shot. I'm sorry you're in this situation, it's very upsetting and sad.
