If you mean broken like it won't drain then it's very fixable. If broken like damaged where you have pieces of the sink broken then your going to need to offer to pay for it by doing extra chores or earning money somehow. If it won't drain you can use what is called a plumbers snake and unclog it.
Honesty is a very big word and it will save you a lot of trouble. I suggest that you talk to him about this because he is bound to find out after all. Let him know that it was an accident and that you never meant for it to happen.