Having a degree in special education, I would totally ask your current preschool teacher their personal opinion. If he is emotionally and socially. physically, academically ready, then I say go for it (as long as his preschool teacher ok's it- since you seem concerned about it). You know your child the best, more than anyone else in the world and follow your instinct on this one. You will know right away if the time is right or not. Does your son succeed and is progressing well and normally in preschool? If he is on target with everything according to his preschool teachers, then, in my opinion I would put him where he belongs, which would be to move on to the next step and go to kindergarten. Why hold him back, if he is not behind or showing any signs that he is not ready? If he is ready, then I say go for it, as long as you are comfortable with it and feel it is the right choice to make for your son,
Also, to put your mind at ease, I was young for my grade when growing up and I never ever noticed any maturity gaps with my peers. There was never a problem. I was always on the same level as my peers in my classes and with my friends in the same grade. One year apart as children grow up, TRULY does not make a huge difference, as long as your son meets all the school requirements to be ready for kindergarten at a young age.