I would talk with his current preschool teacher and see what he/she thinks. My 5 year old DD (will be 6 in March) is in kindergarten this year and not doing so well. Her preschool teacher had marked her behind in some areas but I didn't really think that much about it because the school was very academic. I thought they were just advanced and thought she would be fine in a regular public school. She has struggled to recognize letters and her teacher has recommended me using flash cards with her at home. I have quizzed her and quizzed her and she is finally catching on with letters. Now I find out she is really behind on counting. I have begun helping her with that at home but she just doesn't seem to be getting it yet. Knowing what I know now I really wish I would have waited a year. I am an involved parent and I volunteer in both my DD's classes, read to them, and take them places like the children's museum, zoo etc. But she really needs more study help at home than I have time since DH and I work opposite shifts and it is hard to keep little sister occupied to find time to study with DD. Emotionally and socially she seems to be doing fine just not getting the academics.
But having said that I am not to concerned about starting little sister in kindergarten at age 5. She is doing well in preschool now and seems to catch on to things faster than her big sister. She has one more year of preschool before we need to make the decision and I will talk with her preschool teacher before we enroll her in kindergarten to see what she thinks but if they say they think she is on track she will start at the usual age.
But having said that I am not to concerned about starting little sister in kindergarten at age 5. She is doing well in preschool now and seems to catch on to things faster than her big sister. She has one more year of preschool before we need to make the decision and I will talk with her preschool teacher before we enroll her in kindergarten to see what she thinks but if they say they think she is on track she will start at the usual age.