My son uses Minecraft on his iPad. He has a lot of worlds. All of a sudden all of those worlds are gone. When I go to storage on the iPad I see the size of the game is 4 gigs, the worlds are still there they just won't show up when you go into the game to play. He is using 1.16. I did offload them all and reinstalled. Same result. No worlds.
Since you're using Bedrock Edition, you may have logged into a Microsoft Account/Xbox Live account when your son first started playing. Are you still logged in? If you aren't, try logging back in. If you have saved your worlds to Xbox Live, then when you log back in, the worlds might sync back to your device. (Source)
If that doesn't work, someone managed to recover their Bedrock Worlds on PC. You could try following the instructions, but in your case, you will have to use a file explorer to navigate to Apps/com.mojang.minecraftpe/Documents/games/com.mojang/minecraftWorlds/ instead of his PC filepath. To do so, read the following on iExplorer and MCPEedit.
I hope this helps. Good Luck!