Codes of Diagnoses??

Hello all I am just begining my journey, I am getting my letter ready for the Ins. Co. I have been through this before with them, for my son, he had WLS in May 2004, he was denied to the very end, at the external reveiw it was reversed, so I think I know what I am up against. BTW he is doing great down 133 pounds, getting healthier! My question is the diagnoses codes I see written in some posts and on some sample appeal letters, Is it --better to use them and why--- and also where do you find them and know that they are the correct ones for your own situation? Also I am thinking of hiring Gary, from his website, I am almost afraid tempt fate and try it again on my own, Any help is appreciated Thank you Trish