NO NO No
Don't retain him in kindergarten unless he is very immature and his birthday makes him very young for kinder, but even then research does not support that decision.
The National Association for the Education of Young Children (March, 2001) endorsed this position statement developed by the National Association of Early Childhood Specialists in State Departments of Education:
Retention is rejected as a viable option for young children . . . perpetuated on the basis of false assumptions as to its educational benefit. . . Although research does not support the practice of grade retention, many educators and parents do. . . The vast majority of control-group studies, which are structured to measure this comparison, come down clearly on the side of promotion. Students recommended for retention but advanced to the next grade level end up doing as well as or better academically than non-promoted peers. Children who have been retained demonstrate more social regression, display more behavior problems, suffer stress in connection with being retained . . .
Do a search on the internet: kindergarten retention and read the research. There is ALOT.
I am a kindergarten reading coach and I used to recommend retention in kinder but now I do not. The research is correct, I have witnessed first hand. There are better alternatives that do not cause long term damage.
Please read the research and think about it.