I would second Victoria's suggestions.
Also talk to the teacher to find out if your son is doing extremely well in other areas also or if it is mainly just reading. Most teachers have GT training and can let you know if this is something you should check into. If it is mainly just reading, take him to the library and let him pick out books that really interest him. This needs to be something he enjoys so he doesn't see it as punishment but a way to explore the world around him.
Now I am going to speak as a mom to another mom. All children are gifted in some way. The reason to have your child in the GT program at school should be because it is the best way for your child to learn. It should not be for bragging rights as a parent.
I have one child that was in the program and one that was not and these are some lessons that I learned both personnally and from watching other parents.
Lessons learned:
1. GT kids usually love the challenge and the academic competition of learning and exploring new things. One thing to watch out for, is it is easy for them to get into the habit of not doing things that they cannot master after only one or two tries. An example of this from our life-- our daughter wanted to learn how to type when she was younger. We bought a typing program for kids and gave it to her. She worked on it for about 15 minutes or so and then asked me to do the typing test for her. She had already tried the typing test for the first lesson a couple of times but couldn't pass it, so she wanted me to pass it so she could go on to the next lesson. I tried to explain to her that is not how typing works. You have to practice until you get it right. They would rather not try something than to be considered a failure at something. As a parent you need to let them know that it is OK to not be successful at everything. No one is perfect.
2. If you really want to drive a GT kid crazy, give them busy work. I homeschooled our daughters one year. My youngest would sit and do all of her math problems with no problems or complaints. The oldest after about 4 or 5 problems she would realize she either understood the concept or I needed to explain it a different way. If she undersood it, she was ready to move on to something else. Frustration with busy work is alot of fun when they start driver's ed and believe that after they have driven once they know all there is to know about driving. It has taken me 30 years to learn everything she thought she knew in 30 minutes. But that is how her brain works.
3. If you have other children be careful in how you talk or brag about your GT child. My youngest is also very smart but she has to work and study hard for all of her grades whereas her older sister barely looks at something and then just knows it. Our youngest thought for a long time that she was dumb or inferior in our eyes because school was not as easy for her. Although we have talked about this on many occassions, she still has self-esteem issues about this sometimes.
4. Get them involved in other things besides academics. My daughter is in the orchestra, takes German, is involved in theater, our church youth group and praise band and most importantly loves to hangout with her friends during any spare time she can find.
5. Finally if they are tested for GT and get into the program you will find that some teachers are great and some are not so great. Our daughter's 3rd grade teachers used unit studies. For example if they were studying Egypt. They read about and did reports on pharoahs. They studied the math and science needed to build pyramids. Then built their own. It was exciting and I was jealous that I never had teachers who taught that way. Other teachers through the years have thought that being in GT meant nothing but a lot of extra homework. One thing that she never got along the way were good lessons in grammar. I talked to one of her teachers about it when she was in jr. high and she told me not to worry about it because they learn it by reading good books. This year she had to write 2-3 essays a week in her classes and she learned that she really did need some grammar lessons along the way. It also showed in her PSAT scores. So that is one of the things she has to work on this summer before she takes her PSAT test for the last time.
I know this is a lot more information than what you asked for but I know it helps me to get insight from someone who has already walked the path that is before me.
Good luck in making the best decision for your child.