Since I don't know the details of this case, it is hard to answer. If you feel strongly that he should go on to first grade, then I would request a conference with your child's teacher immediately. You may want to request that the school's reading coach or team leader also be present. Listen to what they say. If you still feel that you want your child to move up to first grade, then gather evidence proving to the teacher that your child is capable and ready to move on.
If your child end up being retained, ask the school questions like:
Will my child have the same teacher?
What interventions are going to be in place to help your child next year?
If they suspect a learning problem, what testing will be conducted and when?
How can you help your child over the summer and next year?
Also, have a talk with your child. You can start the conversation by asking:
How do you feel about being in Kindergarten again?
What did you like best about it?
What didn't you like about it?
What do you think you are good at when at school?
What do you think you can do better?
How can I help you?
The end of the school year is coming to an end quickly, so time is not on your side. Make the time in your schedule to meet with the school staff at their earliest connivence.