I would get the shot. It's strongly recommended until about 5 years old, then again for the elderly. His little immune system just isn't as ready to deal with stuff, and a flu infection that a healthy adult would get over in a week could put a young child or elderly adult in the hospital!! And it could lead to secondary infections, like pneumonia. I've gotten my kids flu shots every year since my youngest had RSV twice (and he was 14 months when he got it, so don't assume your "safe" yet) and the doctor said it was in his best interest to get EVERYONE in the house vaccinated. My kids have never never gotten sick off the shot, other than a few aches and pains, and they haven't missed more than a day of school at once for being sick -- I would call that effective!!
Just be sure to get the shot for you little one at a the pediatricians office or a play they recommend - to make sure that the people giving the shot know how to dose it, etc, for one so young (I have no idea if it's any different, but I wouldn't take any chances).