lissener and Sampiro both had wonderful things to say, and I agree with them that your preparing yourself early to unconditionally love and support your child through a possible coming out speaks volumes about your character and is so, so encouraging.
It was said partly in earnest, but I’d like to suggest you also look into information for yourself (just to begin with) regarding gender-variant people. There is a very broad spectrum of people who transgress gender norms, ranging from transsexuals to genderqueer people to garden-variety femme men and butch women, to people who simply experiment with cross-gender behaviour when they are trying to figure themselves out.
These are issues that can be confusing and unfamiliar for many people, especially parents. Based on what you’ve told us, it may be something you want to be read up on, should your son ever tell you he is questioning his gender. In general, though, in such a case he would need the same thing you are already prepared to give him: unconditional love and acceptance of him as a person, and an environment containing positive messages before the issue even arises.