Correct me if I am wrong here msg board but I believe TAG is a generic term standing for Talented and Gifted program. So my question is how did he get identified for TAG. Was it with the school district? And if so then they should have a program of some sort. Of course this depends on your state and the requirements which is where I would look first to see if TAG is mandated in your state. If it is than no matter how small your district is they should accommodate in some form or fashion. THIS said it does not mean that you will get your needs completely fulfilled. You just have to cruise this board to see how hard parents have to battle for their gifted kids.
But if there is no organized program or specific teacher who specializes in TAG than having your son in 'TAG' really is mute. I think IMHO that your son has been identified as gifted period with or without TAG sources and you will need to battle the school and possibly the district to have his needs met. Start with his teacher and see what she suggests if she is even willing to work with you. Ask for a conference with teacher, counselors, and principal to map a path which might include a pull out program, acceleration, etc.
Also did your son take a test? The results will help guide you and the school in what he does and doesn't need. Other possibilities is depending on the results of the test you could submit him to the Davidson program and they will provide you with a non partial advocate who helps you with the school.