Ask what testing is available to determine your son's readiness for first grade. The school should be able to give your son some achievement screening tests to determine his level of reading and math achievement. They may also have some developmental testing available (the Gesell School Readiness Test is one example) that could be given to determine your son's readiness for first grade.
Research tells us that any retention increases the likelihood that a child will drop out of school when he or she reaches the legal age to do that. If a child is to be retained, however, kindergarten and first grade are the best years for that to happen.
If you move to your new home before school is out, go to visit your son's new school. Talk with the first-grade teachers and observe in their classrooms. Even though the children will be a year older than your son, you will get an idea of the expectations that the first-grade teachers have of their students. If you aren't able to visit before the end of the school year, visit the school in the summer and talk with the principal about your son. Whether you decide on kindergarten again or first grade, you will want to ask for the best classroom placement to meet his needs.