Mainly because most of his friends are a year older than him.
My wife didn't enjoy high school that much, she is onboard with early graduation. I had a lot of fun my senior year and I think this kid needs to mature more before going out into the world, so I'm generally against the early graduation. But I won't tell him he can't do it. We are preparing for college applications this fall.
Life is short. If you are not going to be miserable for some reason, I say take it easy and have a fun senior year.