You need to find out why your son does not have the skills to pass third grade. If he actually has a learning disability, the school will work closely with you to develop an Individual Education Plan (IEP) to meet his needs.
In any case, you need to be working right now on ways to improve your son's academic skills. A good place to start is by having the third-grade teacher identify your child's weakest academic skill. Then focus on working with the teacher to improve that skill. Find out what you personally can do. Tutors are a good source of help. They can monitor your son's progress and share the results with his teacher.
You have almost six months to improve your son's skills so that he can handle fourth grade. Ask your school for this time period before a final decision on retention is made. Find out about different learning programs offered in your area that can help him. Your local university, library, museum, and a learning center are some good places to help you begin your search. Consider summer school. There are even summer camps that incorporate academic learning into their camp programs.
Besides focusing on your son's academic problems, try to help him become more mature through assuming certain responsibilities. He needs to have specific chores that he must complete around the house.