Same story as ours. My son starting using the potty for number 2 when he turned 5 (number 1 at age 3) and it was his decision that he will go on his 5th birthday. All kids are different. Rewards, bribes, punishment, badgering, and pushing often does not work with strong willed and determined kids. It is best to leave them alone and let them come up this idea. Potty training is really physical readiness, we don't train kids for that. Your son has already accomplished physical readiness so compliment him for that. A lot of kids who may seemingly be fully potty trained at age 2 or 3 but often have accidents in underpants and wet bed at night even at age 7 or older. So again, your son deserves compliments for not doing that. And going in diaper or potty is simply a matter of choice. Many kids are not ready for big potty just as they are not ready to leave bottle, breast, blanky, or other reminders of babyhood. Don't we like to hang on to our favorite things? Okay agreed that we gotta prepare kids for this big world out there that has already decided (with limited research) what should be done by what age.
So, simply, tell your son once that kids his age should be going in potty and it would be good if he decides to do that too. Tell him the benefits of going to potty - no dirty diapers, no too much poopy butts, can do it all by himself, and so on. And tell him to let you know when would be good time for him to start doing that. Birthday or another special day. Let him choose it and have him mark the calender. It should be his decision. One gentle reminder closer to that date would be ok. And everytime he achieves something or does something else, it is better to say you must be proud of yourself, you did it all by yourself, rather than I am so proud of you. It sounds like as if it is our achievement than of our kids.
Another idea would be to have your son make this decision with his dad. Tell your husband that you plan the whole thing as a secret to surprise you. We use the secret and surprise trick in many other areas such as brushing, eating, getting ready, etc. For kids, everything has to be a game or fun and that is what life is about. No boring lectures about what is right and wrong and we should do it just because rest of the world is doing it this way. Right? Wrong :) The independence to choose and develop ability to think and make own decisions is far more important than following orders. Okay, I should stop lecturing too. Take it easy, good luck, keep up the good diet with lots of water and fiber.
-Rachna