Ready to potty train?

My son is 16 1/2 months old. I have relatives who say he is ready to be potty trained, and others say to wait until he's three. I'm torn between the two. You see, when he poops, he knows he is going to. He stops what he is doing, and either turns his back to everyone, or squats. Then he immediately wants his diaper changed. In the bathtub, he scoots on down by the drain, squats and poops, and then says "bye bye" (like it's actually going to go down the drain). He likes to sit on the toilet, and seems interested. But I just don't think he understands the concept of going on the potty. Anyone have suggestions or been in the same boat? Thanks!