How to get out of a birthday party you don't want to go to?

There is a kid in my cl that always poops his pants, eats his boogers, pees on his shoes, plucks his eyelashes, never showers, never washes his hands, smells terrible, and makes himself barf and he invited me to his birthday party. He is very easy to upset even if your not trying. He cries all the time. I don't want to hurt his feeling but I don't want to upset him. His party's on saturday so I need help fast.