If they can find him and he will fit into their mouths, they will eat him. You might try adding some extra plants/decor to the tank (floating plants work well). If you are intent upon saving him and you do not have an extra tank handy you can pick up a breeder net that will fit into your existing aquarium pretty cheaply. Since there is only one, he would be fine if left in the net for the 2-3 weeks that it would take to get him to a size at which he will not be eaten. Just make sure that he is larger than your largest fish's mouth before you release him. A good quality flake or other fish food ground into a fine powder fed three to four times a day will be sufficient if you don't want to mess around with hatching brine shrimp.