Why is my dog eating everything he sees?

I have just baked a cake because I am having guests tonight. I left the cake on the table, not on the edge, more in the middle of the table. When I came downstairs I found an empty plate and found my dog eating the whole cake! He eats EVERYTHING. He is actually able to jump on the table and eat from our plates! But he only does that when nobody's looking. When we're around he just beggs for food but we always give out to him when he tries to take something from our plates. He is fed regularly, with Royal Canin food. But he ALWAYS seems to be hungry! He eats anything he finds on the streets, so I have to put a muzzle on him when going out for a night walkie so that he doesn't eat anything because then he poops in the house. He is NOT a puppy anymore. He is about one year old or even more. HELP PLEASE! If he doesn't stop that, my parents are threatning to give him back to the dog pound!!!!!