Each child will do it when he/she is ready. I don't think any child should be forced to before he/she is ready, because it just turns into a power struggle. And let's face, you may be able to make a kid sit on the potty, but you sure can't make him go! : )
It isn't good for a child emotionally or psychologically to be forced to train before they're ready.
As for my children, I tried to get my son trained when he turned 2. He tried a couple times, and then he looked at me and said, "Mama, I don't want to go on the potty. I'll do it when I'm 3." So, I said, okay. And we waited til he was 3. About a month leading up to his third birthday, I kept reminding him at each diaper change that pretty soon he would be three and have to use the potty. On his birthday, I said, "Okay, you're three now. You need to pee and poop on the potty from now on." He agreed. And from that day on, he used the potty. (Still wore diapers overnight til he was four, though.)
As for my daughter......one day I couldn't find her. I started freaking out, wondering if she had somehow gotten out of the house. I was running out the house calling her, and finally my son said, "Mom, she's in the powder room." Sure enough, I looked in the powder room, and there she is, sitting on the toilet. (She was so tiny - to this day, I have no idea how she got up on the thing!!) She was 22 months old. So, from then on, I just let her use the bathroom, since she seemed to want to. When she was 2 1/2, she took off her nighttime diaper and threw it and said, "No baby diaper! Me big girl!" And that was the end of diapers in our house.
I realize that I got extremely lucky with not having to train my children. But don't be jealous, because I work in childcare, and have had to train many others!! ; )
If your son is interested, by all means, let him do it. But if he reverts, just go with it. Don't keep pushing just because he has been doing it, and knows how. He WILL start using it again eventually.
Good luck!