I don't like generalized answers. It really depends on the child: personality, LOG (Level of giftedness) and parents' comfort.
If his teacher is suggesting a skip you might want to look at the IOWA Acceleration Scale. Many gifted children are accelerated whole grades. Some, depending on their LOG, are accelerated more than one grade. The point to all of it is to challenge them. There is a possibility that being left in the age group grade can lead to boredom and distaste for school in general. And this is a huge problem later on in their school career if you are trying to wait for the advance classes.
