Firstly, stop freaking out yourself- You've done everything right so far from the sound of it, now we just need to figure out exactly what's happening.
He's definitely freaked out- Can you leave him alone somewhere cool and dark for a few hours? If he's panicked, it may be an idea to let him calm down.
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Then there is finding out why he's floating- it may just be he's gulped a helluva lot of air when he was freaking out, he may have a tummy upset (Has he been eating properly recently?). I wouldn't try feeding him again for at least a few hours, but then it might be an idea to try some blackworms if you've got them, or earthworms if you don't.
The next move once he stops freaking is to monitor the wound- if there's still blood supply to the limb (as the pics seem to suggest) it should start healing, but fungus is still a risk
And finally, DO NOT resolve yourself to the worst yet. Be prepared, yes, but do what you can to pull him through- From the look of him, he was in excellent health beforehand so his chances are pretty darn high.
Any other suggestions, people?