Hi kbod
He is a lucky little fish indeed. Putting in the bogwood and java moss was a good thing. It will probably have some beneficial bacteria on it, even if most of what was in the tank has died already, and you won't need much to keep up with the small amount of ammonia he will produce. I doubt if you even need filtration as much as a little bit of water movement to provide oxygenation.
He could well be hiding in the shade of the wood and java moss, so just let him be for now. You might not even see him again for a while. Just be sure to do a little water change and keep the water fresh. Let him find food on his own until you are sure he's going to eat it rather than taking a chance of adding food that will just spoil.
What I mean by "filter squeezings" is the residue that comes off a filter when you rinse it out in a bucket of used tank water during regular cleanings. You don't have to add any if there were fish in the tank before since there is just him. There should be microorganisms in the gravel that he can eat, so don't clean it until you are sure he's able to eat the food you will provide.
At his age feeding him is going to be guesswork. If this was something you had planned, you would most likely have set the tank up with an easy to clean sand bottom, be giving the batch of fry more substantial food by now, and doing daily water changes. You would be able to see them and monitor their growth and the state of the tank. In his case, however, avoiding harm is going to be your first priority.
He's still tiny and won't have much color. The best way to look for him might be to stare at the tank and let your eyes catch any sign of movement. Of course, like older corys, the fry spend a good part of their time just sitting still, so this won't be easy either.
All I can think of to add is to wish you good luck and urge you to have patience. I truly hope you can raise him.