Probably the person best qualified to answer your question, Carmen, is your son’s kindergarten teacher. Although the transition from kindergarten should be smooth and seamless, it often doesn’t work out that way.
For a long time elementary placement was guided by a “non-graded plan” that would allow for big individual differences—all entirely normal—among young children in the ages at which they achieve various academic skills. Close monitoring of progress at each grade level, including kindergarten, has now tossed the non-graded plan in the trash bin. It has also forced kindergarten teachers to be more concerned with early phonics and printing than with speaking, listening and essential social skills.
In trying to arrive at a decision I would consider your son’s entire development, including size and rudimentary physical skills, like tossing or catching a ball. If he is very small, an extra year to allow him to grow in stature and master a few more physical skills might be very helpful. If he is already tall and well-coordinated, a second year in which he would tower over the other children would probably not be a good idea. Whichever decision is made, I would enroll him in a summer program that would allow him to continue working on his writing and early reading. A summer might be as long a catch-up period as he would need.
Finally I must comment that this decision might not rest in your hands. The school personnel might insist that it is their prerogative to say which grade he should be assigned to in the fall. If that is the case and you disagree with the arrangement, you always have the option of enrolling him in another school.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.