During your child's check-up, talk with your pediatrician about your child's developmental readiness. Signs of readiness include imitating you in the bathroom, coming to you to say he's "going" or he has to "go," understanding two step commands (for example, "go to your room and get your shoes"), putting on and taking off his own clothes, and staying dry for several hours.
Even though your son doesn't sound like he's quite ready to potty train, it's still important to introduce the potty chair and have him become familiar with it. Place it in a convenient location (which doesn't have to be in the bathroom) so that he begins to feel comfortable with it as you encourage him. Also, allow him to watch his parent(s) use the toilet and introduce simple matter-of-fact words for anatomy, as well as urine and stool.
Toilet training is a gradual process that can take up to three months or more. As a parent, you can explain what needs to be done, but it's your child's determination that's necessary. When he's ready, it will seem to happen overnight. Your role as a parent is to encourage, reinforce, and praise your son as he is achieving this developmental milestone.