I don't know anything about Minecraft, but my 6 year old son loves it. Right now he says he flew away and cannot locate his world. He is very upset and is searching all over. Is there any way to find your world if you have explored and gotten lost? We are using a Kindle Fire.
I assume that by "world" you mean a specific location in the world, the main base/house probably.
Pocket Edition worlds aren't terribly large so any place with distinctive above-ground features or nearby landmarks should be fairly easy to find by flying around. Searching the whole world in flight should only be a matter of minutes.
Failing that your son might consider killing himself (in the game). That would cause him to respawn at the last bed he slept in if it is intact or at the initial spawn, might be easier to find his base from there. If he has gathered any resources that he would hate to lose he could place a chest to stash them in and put up a marker to find it easily (a tower of cobblestone with torches high on it is simple and effective for that). Building a similar beacon close to home can help prevent getting lost in the future.