Kindergarten has changed a lot from when we were in school. Most kids no longer nap and academically they are much more advanced.
Try to put emphasis on reading skills. This includes recognizing letters, phonics and sight words, and beyond, if your child is motivated. Reading seems to be the biggest difference and can be the most noticeable deficit in kindergarteners.
Also, help him to develop strong social skills that foster cooperation, sharing, manners, and patience. Kindergarteners are very self-absorbed individuals, and typically their biggest adjustment is to the environment and new routine.
For a lot of children, going to kindergarten outside of the home makes for a very long day. Most kindergartens are a full day, where preschool typically is not. This can make for very tired, cranky little ones as they adjust to this new schedule. As you get closer to the start of kindergarten, start to lengthen your child’s day. If still napping, start to wean your little one down from this part of the routine, or you can push it back to later in the day as if they were coming home from school.