M.D.
I had a cousin sort of like that growing up, he didn't like loud sounds, and this one party place for kids had a horn, he would always cry and when I was little I would say he would "ruin" my birthday, just too young to really understand. It could be a couple of things, the attention he's getting, or he could have a sensory disorder that loud noises affect him.
Ask him if everyone just says "Happy Birthday" if that bother's him, then everyone could say it and he could blow out his candles...
Happy Early Birthday to Him...