My cat won't stop begging for food

It's all in the subject title. To give you a feel for things, my cat
will beg (cry, poke at me, search the trash for food) as soon as I get
home. I give him a 3oz can of gravy-lovers food, and a bowl of hard
food, and he licks up all the gravy in the first and takes a nibble
from the second, then comes back and cries, pokes and searches,
again. If I give him more, he repeats the pattern. He leaves all the
food in the bowl, but still wants more.

He is now up to 14 lbs, and I refuse to give him more until his late
night feeding, but this goes on all day AND NIGHT. He wakes me at
least 3 times a night. I can't see why he wants more food when he
leaves so much behind.

What do I do? I'm really frantic about this.