One factor is the reaction of others in society to their decision. Many times parents fear of the negative reactions of their friends, relative, peers or school administration effects their decision to hold the child back one year. Most people would agree that the most important factor should be that the child progress normally and live a normal life. The temporary reactions of others will not be remembered in most cases once the child is making wonderful grades and living a normal childhood with no behavior problems.
A second factor is the actual work that the child does based on the decision that is made. In some cases people will say that he will not be challenged as much if he stays in the same grade. When in fact usually reviewing the same material with basic academics can build a strong foundation for future academics.
A third factor to consider is the social relationships of the child. If the child is very young usually the social factors that are disrupted by keeping the child back are only temporary and are forgotten in the next year or so. The friends he would have had in kindergarten are just replaced by different friends when he takes the class again.
Finally, maturity, height and biological issues sometimes discourage parents from holding a child back. When in fact a child that is taller or smarter than the others will usually receive more accolades and awards and positive feedback then the child that is the shortest and not as knowledgeable in the classroom. The positive feedback can lead to an improved self-concept and higher expectation for success that usually lead to a what is referred to as "positive momentum" and further success for the child.
In general, each of these factors should be weighted and if there is more doubt then it is the current authors opinion that keeping the child in the current classroom with ABA assistance will lead to a more productive life. The past problems will be forgotten once the child is receiving positive feedback and success. The child will more probability that he will be mainstreamed and graduate in the regular classroom, go to college possibly and live a much more fulfilled life in the future.