1) He's a busy as a bee. This means to be very busy or active. It doesn't always mean hardworking; it could simply mean the person does many things. 2) He works like a beaver. This means to be very hardworking. "Eager beaver" also means a very enthusiastic worker.

We can say someone is "busy as a bee." I have heard the CEO of a big company talk refer to all of the non-managers as "worker bees," and I thought it was belittling and didn't like it. People who do not do much work are sometimes called "drones" (it's insulting, of course). The most common animal analogy for a hard worker is "he works like a horse."

Helen The others have given good answers: bees, horse and beaver. This is a high risk area for learners. It is too easy to cause offence or misunderstanding in the wrong context (see Dan's answer). If I were you, I would not use any of those similes.

"Works like a bee" is a common phrase for this situation.. although "bee" is usually replaced by an expletive. "works live a beever" is also common and similar.