Argh, annoying! Do you reckon he took an inventory of the eggs? As in, leave a few there under the bed, hide the rest somewhere else, and say that the ones under the bed are from you, like Emmy says.

Another option is to tell him the truth and say that if he tells his sister, then she will get all his eggs. You only get to eat eggs if you believe in the easter bunny!
That worked for us as kids, mum would say (knowing full well we didn't believe in Santa) 'only kids who believe in Santa get presents - do you believe in Santa?' People who say he's not true won't get presents, and we knew we had to toe the party line for pressies!

Good luck mate.