I think there's a difference between being rude, and being assertive, and the trick here, perhaps is for you to start doing things which help you develop more self confidence. Local Authority Evening Class Brochures for the coming September will be coming out soon. Why not get one from your local library and book yourself on a Confidence Building or Self-Assertiveness course, or even Public Speaking. The thought of doing something like this may be rather scary, especially if you admit to being a little shy, but everybody on these courses will be attending for the same reason and you will find yourself in a supportive environment. It will also get you used to hearing your voice spoken out loud and being able to project it in a confident way.
In the meantime, why not start practicing saying aloud in front of a mirror "I really don't know why you are always so rude to me. I find it very offensive and intimidating". (or words to that effect. Then, one day when your boss is very rude to you, say the words out loud so that everybody can hear you. He may be so shocked that "the worm has turned" that it mightcause him to rethink his attitude. And if you include the words "Intimidating", he may start to wonder whether he could be putting himself in line for a case of harrassment.
If your colleages at work are supportive, could you share your problem with them and role-play with them in a quiet place?