Unfortunately, that one short attack by the blackbirds did too much damage. It was obvious that he wasn't going to make it so I put the little guy in a protected spot in the shade and he passed very quickly.
I did call the local shelter and they basically said that nature had to take its course. In hindsight, I wish I had moved him to a safer spot earlier but the parents were being so vigilant. I'm surprised that they didn't protect him from the blackbirds. Maybe there was something else wrong and they knew better. The crueler side of nature is hard to witness. Thanks, everyone, for the advice.