Was it in writing?

or in your original offer letter did it say you are guaranteed a bonus? If not to both questions then treat it as a lession so next time you know to get such an agreement in writing.

You can call him or write to him to see what happens but without a firmal written agreement, there's nothing you can do.

Remember whenver there's money involved, have it in writing. You probably noticed that every ompany would send a formal offer letter and also require a formal resignation letter. It's for a reason.