I would advise taking him to a wildlife rehabilitation center- there is very little you can do on your own. All birds need different food based on their species and it's hard to know what without being an expert.

I hate to tell you this, but even with professional help, there's not much chance he will make it. Injuries from cat attacks are some of the most deadly for animals, because of all the bacteria in cat's mouths.

Also, even if you do manage to keep him alive, there's nowhere to go from there- he won't learn to be completely wild on his own, and won't make a great pet (and it might be illegal). Good luck!

Source: Former wildlife rehab clinic employee